Heavy water distillation

Meier, W., et al. Sulzer Experience with DW Systems, Paper presented at AECL Symposium on Heavy Water Distillation, Apr. 1976. 

Monodeuterated water HDO has an abundance in natural water of approximately 600 ppm. One of the methods used in early attempts to separate the two isotopes was fractional distillation, for which the separation factor at 1 atm was found to be 1.026. Although this procedure was ultimately superseded by the more efficient chemical-exchange processes for larger-scale production of heavy water, distillation remains the separation method of choice for upgrading small amounts of heavy water that have been contaminated by atmospheric water vapor. Distillation is in this case carried out at reduced pressure to take advantage of the higher separation factor. Suppose we wish to carry out the distillation of H2O-HDO at ambient temperatures, i.e., at subatmospheric... 

In the heavy-water plants constmcted at Savannah River and at Dana, these considerations led to designs in which the relatively economical GS process was used to concentrate the deuterium content of natural water to about 15 mol %. Vacuum distillation of water was selected (because there is Httle likelihood of product loss) for the additional concentration of the GS product from 15 to 90% D2O, and an electrolytic process was used to produce the final reactor-grade concentrate of 99.75% D2O. 

Heavy water [11105-15-0] 1 2 produced by a combination of electrolysis and catalytic exchange reactions. Some nuclear reactors (qv) require heavy water as a moderator of neutrons. Plants for the production of heavy water were built by the U.S. government during World War II. These plants, located at Trad, British Columbia, Morgantown, West Virginia, and Savaimah River, South Carolina, have been shut down except for a portion of the Savaimah River plant, which produces heavy water by a three-stage process (see Deuterium and tritium) an H2S/H2O exchange process produces 15% D2O a vacuum distillation increases the concentration to 90% D2O an electrolysis system produces 99.75% D2O (58). 

The extraction of deuterium from natural water feed forms an excellent case study of the application of large scale distillation and exchange distillation to isotope separation. The principal historical demand for deuterium has been as heavy water, D20, for use in certain nuclear reactors. Deuterium is an excellent neutron moderator, and more importantly, it has a low absorption cross section for slow neutrons. Therefore a reactor moderated and cooled with D20 can be fueled with natural uranium thus avoiding the problems of uranium isotope enrichment. This was the... 

Because of the very large enrichments required in heavy water production, cascades taper markedly. In the upper stages the relative advantage of chemical exchange over water distillation vanishes. Most heavy water plants carry out the last portion of the enrichment by distillation (from 20% or 30% D to 99.85%). Accordingly both exchange and distillation will be briefly treated below. First, however, to clarify the important distinction between chemical and thermal reflux we treat an example of isotope separation using chemical reflux. 

Preparation of 1-Octanol-d, CH3(CH2)6CH2OD A mixture of 4.87 g. (0.0375 mole) of octanol-1, 15.38 g. (0.69 mole) of D20 (99.8% purity), and 0.1 g. of anhydrous potassium carbonate was placed into a 50-ml. glass-stoppered flask, shaken for 15 min., and allowed to stand for 60 hr. After an additional 1-hr. shaking period, the two phases were separated and the alcohol layer was dried over anhydrous potassium carbonate. The water layer was distilled in a small distilling flask. Density determination of the original and the reacted heavy water indicated that 92 + 4% of the 1-octanol had been converted to 1-octanol-d. 

About 10 g. of pure sulfur(VI) oxide is prepared in the apparatus shown in Fig. 13. Concentrated oleum is placed in flask A which is cooled, evacuated, and sealed at point 1. Tube B is cooled in liquid air until the required amount of sulfur(VI) oxide has distilled over it is then sealed at point 2. Further purification of the trioxide by vacuum distillation is usually unnecessary. The procedure for reaction of the oxide with the required amount of heavy water (2.5018 g./lO g. SOs) is similar to that described in the preparation of deuterophosphoric acid except that sulfur(VI) oxide, being more volatile than phosphorus (V) oxide, should be cooled to —78° before being placed in the side arm of Fig. 14. An inert Fluorlube tap grease is recommended. Limb E is cooled and the apparatus evacuated slowly to prevent volatilization and entrainment of sulfur (VI) oxide in the vacuum line. When the oxide has been completely... 

When the crucible has cooled down, the system is thoroughly evacuated, and water, or heavy water (ca. 10 ml.), allowed to drip slowly onto the alloy. The progress of the reaction can be followed by observing the manometer. When no more gas is formed, the products are condensed successively in the traps 1, 2, and 3 and in the collection vessel. The tap A is closed when the pressure drops below ca. 10 mm. to keep to a minimum the amount of water that distills over. The reaction vessel is evacuated, filled with N2 or Ar, and removed to a fume hood. 

Derivation Electrolysis of high-purity heavy water, fractional distillation of liquid hydrogen. 

Distillation can be used for the final enrichment, or upgrading, or extracting of light water from the moderator heavy water. 

D2O-DT exchange can be used for transferring tritium from heavy water to deuterium. Further enrichment is achieved by cryogenic distillation. Because of the similarity between deuterium and tritium, platinum on charcoal is the catalyst for vapor phase exchange, whereas hydrophobic catalyst is used for liquid-gas exchange. 

Vapor phase catalytic exchange (VPCE), followed by cryogenic distillation Platinum on charcoal catalyst is used for the exchange. The heavy water has to be converted into steam. The exchange is, therefore, carried out at 200° C. 

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