7.1 General Description

When heat flows through a mixture initially of uniform composition, small diffusion currents are set up, with one component transported in the direction of heat flow, and the other in the opposite direction. This is known as the thermal diffusion effect. The existence of thermal diffusion was predicted theoretically in 1911 by Enskog [El, E2] from the kinetic theory of gases and confirmed experimentally by Chapman [Cl, C2] in 1916. It is not surprising that the effect was not discovered sooner, because it is very small. For example, when a mixture of 50 percent hydrogen and 50 percent nitrogen is held in a temperature gradient between 260 and 10°C, the difference in composition at steady state is only 5 percent. In isotopic mixtures the effect is even smaller.

Thermal diffusion column. Thermal diffusion remained a scientific curiosity until 1938, when Clusius and Dickel [C5] developed their thermal diffusion column, which made possible useful separations in simple equipment. In the Clusius-Dickel column the mixture to be separated is confined in a long, vertical tube, cooled externally and heated internally by a hot wire at the axis of the tube. Other workers have used the annular type of equipment shown in Fig. 14.38. In both types, the mixture to be separated is confined in a narrow space between an inner heated and an outer cooled surface. The outward flow of heat sets up a small difference in isotopic composition through the thermal diffusion effect, with the light isotope usually concentrating in the inner zone at the higher temperature. At the same time, convection currents are set up, as indicated by the arrows, with the lighter heated fluid adjacent to the inner wall moving upward and the heavier cooled fluid adjacent to the outer wall moving downward. This counterflow multiplies the small composition difference obtained from the thermal diffusion effect and makes possible substantial degrees of separation in a practical length of column. For example, in a column 36 m long, Clusius and Dickel were able to separate the isotopes of chlorine, producing HC1 containing 99.6 percent 35C1 at one end of the column and HC1 containing 99.4 percent 37C1 at the other.

For most isotopes it is preferable to work with gases rather than liquids, because the higher diffusion coefficients result in higher separative capacity. The optimum pressure is usually near atmospheric. However, when 235U was first found to be fissionable, Nier [N3] attempted to separate it by thermal diffusion of UF6 vapor at low pressure without success, so that it was necessary to work with the liquid at high pressures [Al] to obtain useful separation. The optimum spacing between hot and cold surfaces is a few millimeters for gases and fraction of a millimeter for liquids.

The degree of separation obtainable in thermal diffusion (the difference in composition between hot and cold walls) is much less than in other diffusion processes, so that use of a column to multiply the composition difference is practically essential. The stage type of thermal diffusion has been used only to measure the thermal diffusion coefficient and is never used for practical separations. In some thermal diffusion columns, htu’s are as low as 1.5 cm, and as many as 800 stages of separation have been obtained from a single column. Even with such a great increase in separation, it is often necessary to use a tapered cascade of thermal diffusion columns for isotopic mixtures, to minimize hold-up of partially enriched isotopes and to reduce equilibrium time.

Isotopes separated. Table 14.24 gives examples of some of the highest reported concentrations of separated isotopes that have been obtained by thermal diffusion. Most of these separations were on a small laboratory scale. The high purity to which scarce isotopes such as 13C, 15N, and leO have been concentrated is a notable feature of these examples of thermal diffusion. The feasibility of concentrating rare isotopes of intermediate mass, such as J1Ne and “A, by thermal diffusion is also noteworthy. These separations are facilitated by the large number of stages obtainable from a single thermal diffusion column.

Thermal diffusion is a convenient way of separating isotopes on a small scale. It is a very inefficient process for large-scale use because of its high energy consumption.