Although water distillation is not competitive with other processes for primary concentration of deuterium from natural water, owing to the high energy consumption and the large number of towers used by this process, it is the preferred method for final concentration of deuterium from water previously enriched to several percent deuterium. In this high-enrichment range, energy consumption or equipment size is only a small fraction of that needed for primary concentration, and the reliability and simplicity of water distillation make it the process preferred for final concentration. Almost all final concentration water distillation plants installed since 1960 have been built by Sulzer Brothers, of Winterthur, Switzerland, including the final concentration sections of plants 1, 10, 12, 13, 15, 19, and 20 of Table 13.2.

Water distillation is also generally used for purifying D20 that may have become contaminated in use by H2 О through dilution or DTO through neutron absorption.

The design of Sulzer water distillation plants for these purposes, described in references [B2], [Dl], [H7], [M5], and [Zl], has evolved through several stages. The most recent plants use Sulzer CY packing made of copper, chemically treated to improve wettability. As shown in Fig. 13.10, this packing consists of parallel strips of wire mesh with oblique corrugations, arranged vertically. The slopes of adjacent strips are in opposite directions. The packing is fabricated in cylindrical cartridges about 160 mm high. Successive cartridges are turned 90° from adjacent ones. Because of the 90° displacement and the oblique corrugations, the gas
stream is well mixed at each elevation, and the liquid trickles down in a zigzag motion. After flowing through 3 to 4 m of packing, the liquid is collected, mixed, and redistributed over the top of the next packed section. Because of these means to keep gas and liquid well mixed, no increase in transfer unit height has been noted in column diameters up to 2 m.

Figure 13.11 shows the principal characteristics of Sulzer CY packing for water distillation service [M6]. The optimum throughput is said to be at 75 percent of flooding, at which the F factor is 1.7. At this load, the gas-phase pressure drop is about 4 Torr/m, the liquid holdup is about 6 percent of the packed volume, and the observed height of a transfer unit (htu) has been found to be between 6.5 and 12 cm. The observed variations in htu are attributed to variations in the wetting of the packing, which is impaired by traces of oil and other hydrophobic impurities in the water.

At an F factor of 1.7, the water vapor throughput at several pressures is as follows:

5.2 Separation of 18 О by Distillation of Water

Water distillation has been used by Dostrovsky [D4] to produce 13 g/day of D218 О containing

99.8 percent 180 from natural water containing 0.204 a/о 180. Figure 13.12 shows external flows between stages in this water distillation plant; reboilers and condensers are not shown. Table 13.12 summarizes process conditions in this plant. This plant has the stepped-down tapered shape characteristic of a squared-off cascade.

The columns are packed with Dixon [D3] rings made from 100-mesh phosphor bronze wire gauze. The columns are operated at a mean pressure of approximately 0.5 atm, with a pressure drop of 130 Torr. Under these conditions, Dostrovsky and co-workers [D5] have found the 160-180 separation factor to be 1.0064, and the height of a transfer unit in the larger columns to be about 2 cm.