Deuterium was discovered by Urey et al. [U2] in samples of liquid hydrogen in which deuterium had been concentrated by partial evaporation. Because of the high deuterium separation factor, separation of deuterium by distillation of liquid hydrogen was studied by engineers in Germany [C3] and the United States [M8] during World War II and more recently by groups in the Soviet Union [Ml], France [Al], Germany [L2], Switzerland [H3], England [D2], and the United States [ВЗ, B4]. The main difficulties with the process have been the extremely low operating temperatures, which until recently have been without industrial precedent, and elimination of condensable impurities from the feed stream, which would foul heat exchangers and stop flow if not removed. Because these difficulties are those of low-temperature plants and are not unique to isotope separation, they will not be dealt with

Table 13.5 Deuterium separation factors in distillation of ammonia Pressure, Torr Temperature, °С Separation factor Measured, at 24 percent deuterium From Eq. (13.9) 764 -32.6 1.0429 ±0.0015 1.042 500 -40.6 1.050 ±0.0007 1.047 250 -52.2 1.0564 ±0.0013 1.055

Table 13.6 Effect of deuterium content on separation factor in distillation of ammonia Atom percent deuterium in liquid (760 Torr pressure) 10 24 42 58 Eq. (13.9) Separation factor Experimental uncertainty 1.0435 0.0016 1.0436 0.0010 1.0402 0.0006 1.0383 0.0007 1.042

extensively here. The references cited above may be consulted for more detailed information. Plants producing deuterium by distillation of liquid hydrogen that have been built and operated are listed in Table 13.7. The process used for the primary concentration of deuterium in all of these plants is similar in principle and is illustrated in generalized fashion in Fig. 13.5. The individual plants differ in detail; some of the principal differences are noted in Table 13.7. More detail is given in the references cited in Table 13.7.

The history of these plants has been sketched in Sec. 2.2 of Chap. 12. Each plant is parasitic to an ammonia synthesis plant, taking deuterium-bearing, hydrogen-rich feed gas from the ammonia plant, and returning gas depleted in deuterium to the ammonia plant, with little loss of hydrogen (less than 5 percent). The first two plants listed in Table 13.7 used as feed ammonia synthesis gas, which contains around 75 percent H2, 25 percent N2, and small amounts of CH4, A, C02, CO, 02, and H20. The remaining plants used as feed electrolytic hydrogen, which contains as impurities only H2 О and traces of N2 and 02. The high content of nitrogen and the presence of other impurities in the ammonia synthesis gas used as feed in the first two plants caused their design to be more complicated, their specific energy consumption higher, and the cost of heavy water produced in them greater than in the three plants using electrolytic hydrogen and feed. In fact, the first two plants were built primarily as pilot plants rather than as economic producers of heavy water, and they have been shut down, having served their purpose.

The process used in the primary section of these plants may be understood by reference to Fig. 13.1. Where gas from the ammonia plant is available under pressure, it is fed directly to the hydrogen distillation plant; otherwise it is compressed in the feed compressor. The gas is cooled down to around — 175°C by outflowing cold gas depleted in deuterium in a heat exchange system in which water is condensed and removed from the feed. Refrigeration to compensate for heat leaking into the plant may next be supplied to the feed. The gas is cooled further to about —245°C by outflowing cold gas in a second heat exchange system in which nitrogen is condensed and removed from the feed. Much nitrogen is condensed from synthesis gas; traces, from electrolytic hydrogen. Final cooling is provided by Joule-Thomson expansion through a valve, in which hydrogen is cooled to around —250°C and partially liquefied.

The hydrogen is distilled in the primary tower into a bottom product enriched in deuterium and an overhead product depleted in deuterium. Final concentration of the bottom product is effected by distillation either of liquid hydrogen or water (not shown in Fig. 13.1). The depleted hydrogen flows back through the feed exchanger system where it is warmed to room temperature. It is returned to the ammonia plant at the supply pressure, being compressed if necessary.

To provide heat to reboil the tower and to supply liquid hydrogen reflux, additional depleted hydrogen is circulated by the reflux compressor through another system of heat exchangers, to which additional refrigeration may be supplied. Cold, compressed hydrogen from this system flows through a coil at the bottom of the tower where it is condensed, supplying

Table 13.7 Hydrogen distillation heavy-water plants Plant location Toulouse, France Hoechst, Germany Designer Compagnie Francaise Unde Operator de l’Eau Lourde Fatbwerke Hoechst Year production started 1958 1958 Year production ended Feed gas (l)t 1960 I960 Material NH3 synthesis gas NH, synthesis gas nm’H,/h 3000 6300 ppm D in H 120 105 D recovery,% 65 85 Production rate, kg D, O/day 3.5 12 Energy cons., kWh/kg D, О Pressure, atm 17,200 8000 Feed (1) 230 20 Recycle (2) 2-16 50 Stripped gas (3) 2.5 1 Flow ratio, recycle/feed H, 7 0.43 Method of removing H, О Alumina Regenerators Method of removing N, Adsorption Regenerators Refrigeration Piston expander Uquid N, Stream applied to Primary tower Feed Feed + recycle Type Double Triple Internals l’Air Liquide Sieve plates Diameter, m 7 1.2 Packed height 85 plates 30 m % HD in bottoms (4) 2 4 Material distilled for final D cone. Hydrogen Hydrogen Reference [All [ 1-2] tNumbers are keyed to Fig. 13.1.

Soviet Union Soviet govt.

9

?

Electrolytic Hj

4000

150

?

13

5000

2-4 6 + 70

9

9

Switch exchangers Adsorption

Liquid NH, + liquid N, Feed + recycle

Single

Bubble caps

1.05

77 plates 7-9

9

[Mil

Ems, Switzerland Sulzer Bros. Emswerke AG 1960 1967

Electrolytic H3

400

970

85

7

2400

3.7

14

1.5

7.5

Switch exchangers Switch exchangers Turbine expander Recycle

Single

Kuhn, Dixon 90 tubes, 5-cm diam. 2m 60

Water

[H31

Nangal, India Unde

Indian govt. 1962

Electrolytic H,

5300

450

90

45

2100

5

40

1

1.24

Regenerators Regenerators Uquid NH, + liquid N, Feed + recycle

Triple Sieve plates

9

9

4

Hydrogen

[Gil

heat to reboil the tower at the same time. This liquid is then expanded to tower pressure through a valve and fed to the top of the tower for reflux.

The product of these primary plants is a stream of hydrogen containing from 2 to 60 percent HD. At Ems this hydrogen was converted to water by burning it with oxygen, and pure heavy water was produced by distilling the water. At Toulouse, Hoechst, and Nangal, the HD-rich hydrogen stream was distilled directly to produce pure deuterium, which might then be burned to make heavy water. The basic flow sheet for this final concentration of deuterium was devised by Clusius and Starke [C7], who conducted the first experimental work on the fractional distillation of liquid hydrogen and showed that a mixture of hydrogen, HD, and deuterium could be separated by fractional distillation at atmospheric pressure into relatively pure fractions of H2, HD, and Ds without HD undergoing disproportionation and without appreciable conversion of ortho to para modifications.

The flow sheet for final concentration of deuterium developed by Clusius and Starke, which was used in the Hoechst and Nangal plants, is shown in Fig. 13.2, together with the primary tower of Fig. 13.1.

The bottoms from the primary tower are fed into the upper half of a smaller secondary tower, where fractionation into a bottom product of nearly pure HD is completed. This HD is warmed to room temperature in a heat exchanger and passed through a catalytic exchange reactor where its disproportionation into an equilibrium mixture of H2, HD, and D2 is catalyzed. The product of the exchange reaction is cooled to liquid hydrogen temperatures in the heat exchanger and fed to the bottom half of the secondary tower where it is fractionated into an overhead product of HD + H2 and a bottom product of pure deuterium. This is warmed to room temperature in the heat exchanger and constitutes the product of the plant. The HD and H2 overhead from the bottom of the secondary tower is fed to the top of the secondary tower for recovery of HD.

Heat to reboil these towers is provided by a stream of compressed, HD-free hydrogen,
which is condensed in reboiler coils located in the sump of these towers. The condensed HD-free hydrogen is then used as open reflux in the top of the primary tower. A Linde, double-column arrangement is used to provide reflux for the bottom of the secondary tower and reboil vapor for the top of this tower.

Of the plants listed in Table 13.7, the one at Nangal, India, may be regarded as indicating the full potentialities of this method of producing deuterium. It is a relatively large plant, producing around 141 of heavy water per year. It uses clean electrolytic hydrogen as feed. This hydrogen has been preconcentrated by two stages of partial electrolysis of water to around three times natural abundance. Power costs at Nangal, which is the site of a large hydroelectric project, are low. These three favorable circumstances make it possible to produce heavy water at a specific energy consumption in the distillation plant of only 2.1 kWh/g D20. This is lower than the energy consumption at the other sites, and of course is much lower than the 468 kWh/g D20 for electrolysis alone noted in Sec. 6. Garni et al. [Gl] in 1958 predicted that heavy water would be produced at Nangal at a cost of $27.2/lb or $60/kg. Data cited by these authors in 1958 as typical of what production rate and costs might be experienced at Nangal when the plant went into operation are summarized in Table 13.8.

A special problem of hydrogen distillation plants is the need to minimize conversion of ortho to para hydrogen. At room temperature, hydrogen contains 75 percent ortho and 25 percent para hydrogen. At low, hydrogen distillation temperature, the equilibrium proportion is nearly 100 percent para hydrogen. Conversion of ortho to para hydrogen is very slow in the absence of catalysts. Conversion must be minimized in a deuterium separation plant because about 1.5 times as much heat is released in conversion of ortho hydrogen as in liquefaction; it would greatly increase power consumption if allowed to occur. Conversion is catalyzed by paramagnetic materials, such as solid oxygen, and by ferromagnetic materials, such as certain steels. These must be excluded from the plant.

Condensed oxygen is especially objectionable, both because of the heat produced in ortho-para catalysis and because of its liability to explode when in contact with cold hydrogen.

Table 13.8 Production and cost data anticipated for Nangal heavy — water plant

Stages of electrolytic preconcentration, 2 Hydrogen production rate, 25,000 nm3/h Hydrogen feed rate to distillation plant, 5000 nm3/h Producing hours per year, 8000 D20 production rate, 14,000 kg/yr Erected cost of plant, $2.75 million

Production costs	$ million/yr	$/kg D20
Capital charges at 16.8%/yr	0.462	33.0
Power	0.130	9.3
Hydrogen loss	0.090	6.4
Labor and maintenance	0.130	9.3
Supplies	0.027	1.9
Total	0.839	59.9