Method for separating at least one heavy isotope from a hydrogen-containing medium

The multi-stage process separates at least one heavy isotope from a hydrogen-containing compound or a hydrogen containing mixture, using ammonia synthesis and a mixture of hydrogen and nitrogen. The main product is water at least substantially free of deuterium and tritium, additional products being compounds enriched in deuterium and tritium, and nitrogen enriched in .sup.15 N.

The invention relates to a method of separating at least one heavy isotope 
from a hydrogen-containing medium. More particularly, this invention 
relates to a method of separating at least one heavy isotope from a 
hydrogen-containing compound or a hydrogen-containing mixture using 
ammonia synthesis and a hydrogen-nitrogen mixture. 
A method of separating a heavy isotope from a hydrogen-containing medium is 
known e.g. from the book "NUCLEAR CHEMICAL ENGINEERING", Second Edition, 
by M. Benedict, Th. H. Pigford and H. W. Levi, McGraw-Hill Book Company, 
pages 763 to 765. 
This method constitutes a monothermic ammonia-hydrogen exchange process in 
which ammonia enriched nearly 100% with deuterium is used as a starting 
product for obtaining water. In this case, the exchange process is an 
parasitic process combined with a method of obtaining ammonia from 
synthesis gas consisting of nitrogen and hydrogen having a natural 
deuterium concentration. 
It is an object of the invention, to remove practically all the deuterium 
and tritium from deuterium and tritium-containing water to obtain water 
depleted in deuterium and tritium. 
It is another object of the invention to obtain water free of deutrium and 
tritium. 
It is another object of the invention to provide an economic method of 
obtaining deuterium and tritium depleted water as a main product and 
deuterium and/or tritium enriched by-products. 
Briefly, the invention provides a method of separating at least one isotope 
from a hydrogen-containing medium which comprises the steps of bringing a 
flow of deuterium and tritium containing feed water into a first isotope 
exchange with a flow of ammonia vapor depleted in deuterium and tritium 
with the ammonia molar throughput being greater than two-thirds of the 
water molar throughput and deleting the flow of water almost completely of 
deuterium and tritium during the isotope exchange to obtain water depleted 
of deuterium and tritium as a product while enriching the flow of ammonia 
vapor in deuterium and tritium to obtain ammonia vapor enriched with 
deterium and tritium at a concentration lower than the concentration of 
deuterium and tritium in the feed water. 
Thereafter, the enriched ammonia vapor is liquefied and then separated into 
a mixture of hydrogen and nitrogen. This mixture is brought into a second 
isotope exchange with a flow of liquid ammonia depleted of deuterium and 
tritium to deplete the mixture of hydrogen and nitrogen of deuterium and 
tritium while enriching the flow of liquid ammonia in deuterium and 
tritium. 
Next, the depleted mixture of hydrogen and nitrogen is synthesized to 
obtain the liquid ammonia for said second isotope exchange with one part 
of the synthesized liquid ammonia being evaporated and re-cycled to the 
first isotope exchange. 
The water can come e.g. from a river or can be water from a nuclear fuel 
processing plant or the cooling water or a moderator of an atomic reactor 
plant. 
As is known, tritium is a dangerous radioactive substance, while 
low-deuterium water has been found to promote growth when used in 
agriculture. 
In addition to the substances depleted in isotopes, the by-products of 
isotope exchange processes are other substances containing higher 
proportions of the isotopes than the feed material. A by-product enriched 
with deuterium, e.g. hydrogen or water, can then e.g. form the feed of a 
conventional process for obtaining heavy water, after tritium has been 
removed from the product in known manner. The tritium can be stored or, 
after concentration in known manner, can be used for nuclear fusion 
processes. 
Since ammonia synthesis is used, ammonia need not be removed in the process 
as a product but only the leakage losses are to be compensated when 
operating the process. 
In one embodiment of the invention, the part of the ammonia enriched with 
deuterium and tritium from the second isotope exchange is evaporated and 
brought into isotope exchange with a second stream of water. The water is 
then enriched with tritium and deuterium and obtained as a by-product, and 
the ammonia vapor depleted in deuterium and tritium is liquefied and mixed 
with the ammonia coming from the first isotope exchange before being 
decomposed into hydrogen and nitrogen. 
In another advantageous embodiment, at least part of the liquid ammonia 
enriched with deuterium and tritium is brought into isotope exchange with 
a second hydrogen/nitrogen gas mixture enriched in deuterium and tritium 
and in the process is further enriched in deuterium and tritium. The 
second gas mixture is obtained from decomposition of the enriched ammonia. 
Part of the gas mixture is used as a by-product enriched in deuterium and 
tritium while another part of the gas mixture depleted in deuterium and 
tritium during isotope exchange is mixed with the gas mixture from the 
first isotope exchange which has been liquefied and separated into 
hydrogen and nitrogen. 
According to another feature the method is also used to obtain isotope 
.sup.15 N as a by-product in a more economic manner than hitherto. The 
.sup.15 N isotope can then be used as a cooling gas for gas-cooled nuclear 
reactors or as "blanket" gas for light and heavy-water reactors. 
Hitherto, only carbon dioxide and helium have been used as cooling gases 
and helium as a blanket gas for light and heavy-water reactors. Carbon 
dioxide, owing to its chemical instability under radiation, is suitable 
only in a temperature range from about 600.degree. to 700.degree. C. Owing 
to its solubility in water, carbon dioxide cannot be used as a washing gas 
for water reactors, since it forms a corrosive acid solution. 
Helium is an expensive gas and, owing to its high permeability, requires a 
very seal-tight plant. 
Hitherto, .sup.15 N isotope has not been used industrially since it has 
been uneconomically expensive to produce, e.g., by distillation of 
nitrogen or liquid ammonia.

Referring to FIG. 1, in order to obtain deuterium and tritium-depleted 
water as a main product and deuterium and tritium-enriched water as a 
by-product from water already containing deuterium and tritium, the method 
is performed as follows: 
A feed water, e.g. river water or waste water containing deuterium and 
tritium, is supplied through a line 1 by a pump 2 to an isotope exchange 
tower 3 and brought into isotope exchange in countercurrent with ammonia 
vapor depleted in deuterium and tritium with ammonia molar throughput 
being greater than two-thirds (2/3) of the water molar throughput, for 
reasons as explained below. 
In the process, the water is substantially depleted of deuterium and 
tritium. The water, which is useful for agricultural or industrial 
purposes, still contains dissolved ammonia, which is undesirable for 
environmental reasons, ammonia consumption, etc. For this reason, the 
ammonia is removed in a column 4 adjacent the bottom part of the tower 3, 
the steam required for separation being produced by a heat source in the 
sump of column 4. The heat source can be a coil 5 heated with steam. The 
ammonia-free product is removed through a line 6. 
The ammonia vapor leaving the top of tower 3 has a somewhat lower 
concentration of deuterium and tritium than the feed water, and contains 
water vapor which must not be present in subsequent steps of the process. 
The water vapor is therefore separated in a rectification column 7 
disposed above the tower 3. The anhydrous ammonia vapor is liquefied in a 
condenser 8 by a water-cooled coil 9. 
Part of the condensate is recycled to a line 10 to the column 7. Most of 
the condensate is conveyed through a line 11 by the pump 12 to a cracking 
oven 13 and converted in known manner into a synthesis gas mixture 
(N.sub.2 +3H.sub.2). This gas mixture is introduced into an isotope 
exchange tower 14 and brought into exchange in counter-current with liquid 
ammonia. The isotope exchange can occur only if the liquid ammonia 
contains a dissolved catalyst, e.g. KNH.sub.2 In the process, the hydrogen 
in the gas mixture loses deuterium and tritium whereas the ammonia becomes 
enriched in deuterium and tritium. 
Ammonia is then formed from the gas mixture in a synthesis plant 15. Most 
of the deuterium and tritium-depleted ammonia is conveyed by a pump 16 
through a line 17 to the top of the tower 14. 
The ammonia enriched in deuterium and tritium during exchange with the 
synthesis gas mixture and containing a catalyst in solution is removed 
from the tower 14 through a line 18, expanded in a throttle valve 19 and 
introduced into a concentrator 20. 
The rest of the ammonia formed in the systhesis plant 15 is removed through 
a line 21 with a part conveyed through a line 22 containing a throttle 
valve 23 and expanded in an evaporator 24. The ammonia vapor, depleted in 
deuterium and tritium, coming from the evaporator 24 is conveyed through a 
line 25 to an exchange tower 26 and brought into isotope exchange in 
counter-current with liquid ammonia enriched in deuterium and tritium. The 
resulting deuterium and tritium-enriched ammonia vapor is conveyed through 
a line 27 to a cooler 28 and there condensed by a cooling coil 29 through 
which cooling water flows. The condensate is removed through a line 30 and 
supplied through a pump 31 to the concentrator 20. 
The concentrator 20 comprises a partial evaporator (not shown) which 
produces a stream of catalyst-free ammonia vapor from the liquids supplied 
through the lines 30 and 18, and a condenser (not shown) which liquefies 
the stream of vapor. 
The liquefied ammonia, enriched in deuterium and tritium, is removed from 
the concentrator 20 through a line 32. 
The partial evaporator incorporated in the concentrator 20 also produces an 
ammonia liquid enriched in catalyst, deuterium and tritium, which is 
removed from the concentrator 20 through a line 33 conveyed through a 
throttle valve 34 and expanded in the top part of the isotope exchange 
tower 26. 
The ammonia depleted in deuterium and tritium and containing dissolved 
catalyst is taken from the bottom of the tower 26 through a line 35 and 
supplied by a pump 36 to the exchange tower 14. 
The ammonia stream removed from the ammonia synthesis plant 15 through the 
line 21, minus the part with drawn through the line 22, is removed through 
a line 37 and, via a throttle valve 38, is expanded in an evaporator 39. 
The deuterium and tritium-depleted ammonia vapor is then supplied through 
a line 40 to the tower 3 to initiate the first process step i.e., the 
first isotope exchange. 
The catalyst-free, deuterium and tritium enriched ammonia vapor taken from 
the concentrator 20 through the line 32 is expanded through a throttle 
valve 41 in an evaporator 42. From here, the ammonia vapor is supplied 
through a line 43 to an isotope exchange tower 44. A second stream of 
water, which can come from the same source as the first stream, is 
supplied to the top of the tower 44 through a line 45 and a pump 46 and 
brought into isotope exchange in countercurrent with ammonia vapor, as in 
the tower 3. In contrast to the first isotope exchange, the water becomes 
enriched with deuterium and tritium since the ammonia vapor is enriched in 
deuterium and tritium. 
To separate the water vapor, the deuterium and tritium depleted ammonia 
vapor is introduced into a rectification column 47 and the ammonia vapor 
freed from water vapor is liquefied in a condenser 48 containing a coil 49 
through which a cooling medium flows. Some of the condensate is recycled 
through a line 50 to the rectification column 47, whereas the much larger 
part of the condensate is removed through a line 51 and mixed with 
condensate taken from the condenser 8 through the line 11. 
A column 52 for separating the ammonia dissolved in water is disposed below 
the tower 44, the steam required for separation being produced by a heat 
source in the sump of the column 52. The heat source can be a coil 53 
heated with steam. 
The ammonia-free, deuterium and tritium-enriched water is removed through a 
line 54. The water can be used as a feed for a heavy-water production 
plant or a tritium concentrating plant. 
Referring to FIG. 2, the method may also be used to obtain water largely 
depleted of deuterium and tritium as a main product, hydrogen enriched 
with deuterium and tritium as a first by-product and nitrogen enriched 
with .sup.15 N as a second by-product. 
To avoid repetitions, those parts of the plant in FIG. 2 similar to the 
plant for performing a process corresponding to FIG. 1 are denoted by the 
same reference numbers plus a prime and operate in similar manner to the 
components in FIG. 1. For example, an exchange column 3' is supplied with 
river water or deuterium and tritium-containing waste water, and water 
substantially or completely freed from deuterium and tritium is removed 
through a line 6'. 
As before, the entire plant is independent of a synthesis plant for 
industrial production of ammonia. 
In contrast to the plant in FIG. 1, no catalyst-free, deuterium and 
tritium-enriched ammonia is removed and used to produce deuterium and 
tritium-containing water serving as an exchange agent. In the present 
case, an equivalent amount of liquid ammonia, still containing catalyst in 
solution, is removed through a line 60 connected the line line 18' and is 
conveyed through a pump 61 to an isotope exchange tower 62, where the 
liquid ammonia is brought into isotope exchange in counter-current with a 
synthesis gas mixture (N.sub.2 +3H.sub.2) enriched in deuterium and 
tritium. The ammonia and catalyst become enriched in deuterium and 
tritium, if required turning into pure ND.sub.3 or pure NT.sub.3, whereas 
the synthesis gas mixture become depleted in .sup.15 N. 
The liquid depleted in deuterium, tritium and .sup.15 N in the tower 62 
flows through a line 63 to a concentrator 64, of similar construction to 
the concentrator 20 (FIG. 1) or 20' (FIG. 2). 
Two flows are produced in the concentrator 64. One flow of liquid has a 
higher catalyst content than the flow leaving exchange tower 62. This 
liquid is enriched with deuterium and tritium and may contain ND.sub.3 
and/or NT.sub.3, but is depleted in .sup.15 N or completely free of 
.sup.15 N if required, and after expansion is conveyed through a line 65 
and throttle valve 66 to a concentrator 20', operating in similar manner 
to the concentrator 20 in FIG. 1. 
A second flow of liquid in the concentrator 64 comprises NH.sub.3 or 
ND.sub.3 and/or NT.sub.3 and is free from catalyst as well as being 
depleted in or completely free from .sup.15 N. 
This second flow is conveyed by a pump 67 to a cracking oven 68 and, 
decomposed into N.sub.2 +3H.sub.2 or N.sub.2 +3D.sub.2 and/or N.sub.2 
+3T.sub.2. 
Most of these gases are returned through lines 69, 70 to the isotope 
exchange tower 62, where they are depleted in .sup.15 N and in deuterium 
and tritium, by the previously described method. The gas is then sent 
through a line 71 and combined with the synthesis gas from the cracking 
oven 13'. 
The remaining decomposition products produced in the oven 68 are sent 
through a line 72 to a hydrogen/nitrogen separator 73 of known 
construction. The process occurring in the separator can be performed by 
low-temperature separation by liquefaction and distillation or by 
alternating selective absorption or through a selectively permeable 
diaphragm. 
The gas mixture introduced into the separator 73 is decomposed into 
nitrogen and hydrogen. Nitrogen free from hydrogen and depleted in or free 
from .sup.15 N is removed through a line 74. This nitrogen can be used for 
industrial purposes. 
Some of the nitrogen-free hydrogen, enriched in deutrium and tritium, 
removed through a line 75 can be obtained as product through the line 76. 
The product can then be enriched in deuterium or tritium in a manner not 
shown, through at least one additional upstream stage. 
Alternatively, the product can be burnt with oxygen to form water 
containing deuterium or tritium. The remaining flow is recirculated 
through a line 77 by a compressor 78 and added to the flow from the line 
69, the gas mixture being sent through the line 70 to the exchange tower 
62. 
Ammonia depleted in deuterium and tritium through the line 37' but enriched 
in .sup.15 N is expanded as in the method of FIG. 1 and evaporated and 
brought into isotope exchange with water which may or may not contain 
deuterium or tritium, so that ammonia enriched in .sup.15 N flows through 
the line 11', the deuterium and tritium content corresponding to that of 
the feed water. 
After leaving the tower 14', some of the synthesis gas depleted in 
deuterium and tritium is branched off through a line 79 from the gas 
supplied to the ammonia synthesis plant 15', and the branched-off gas is 
introduced into a known hydrogen-nitrogen separator 80. 
In the separator 80, which can be constructed as the separator 73, the gas 
mixture is divided into two flows. 
A first nitrogen flow, enriched with .sup.15 N and preferably 
hydrogen-free, is withdrawn through a line 81. 
A second gas flow, consisting mainly of hydrogen (but not necessarily free 
of nitrogen) is sent through a line 82 and a compressor 83 and mixed with 
the flow of gas introduced into the exchange tower 14'. 
In this embodiment, nitrogen containing the natural concentration of 
.sup.15 N is introduced into the plant through a line 84, to make up for 
the flow of product taken from the plant through the line 81 and the 
nitrogen taken through the line 74. 
Alternatively, line 84 can be connected to the gas inlet of the tower 62 
instead of to the inlet of tower 14'. The place where fresh nitrogen is 
injected will depend on the requirements of the process, particularly on 
the desired proportions of the .sup.15 N, deuterium and tritium products. 
To summarise, in the embodiment in FIG. 2, the products consumed are fresh 
water or deuterium and tritium-containing waste water and nitrogen gas, 
whereas the main products are water at least substantially freed from 
deuterium and tritium through line 6', nitrogen enriched in .sup.15 N 
through line 81 and hydrogen enriched in deuterium and tritium through 
line 76. The by-product is nitrogen depleted in .sup.15 N, which is 
removed through line 74. 
The consumption of ammonia is extremely small and mainly due to leakages in 
the plant. 
Finally, it may be advantageous to connect a number of identical plants 
according to FIG. 1 or FIG. 2 in series, in order to obtain products as 
required, more strongly enriched or depleted in the corresponding 
isotopes. 
As an alternative to series connection, a single plant can be periodically 
operated, at least one product from one period being used as the feed for 
the next period. 
If the products need to be completely pure, e.g. water completely free from 
deuterium and tritium or pure isotope .sup.15 N, conventional final 
enrichment or depletion stages such as distillation columns can be 
connected downstream of the product withdrawal points. 
Numerical example 
Plant for simultaneously depleting deuterium and tritium and obtaining 
heavy water or water enriched in deuterium. 
Assumed capacity: 
1. Removal of tritium from waste water: 
3000 m.sup.3 /year waste water containing 200 Ci/m.sup.3 tritium, coming 
from a nuclear fuel processing plant treating 1400 tons (t) per year, is 
to be brought to the legally permitted concentration of 0.03 Ci/m.sup.3 
2. Deuterium-free water: 
300 m.sup.3 /h water containing less than 10 ppm D/D+H must be produced for 
agricultural purposes. The tritium concentration must be below 0.03 
Ci/m.sup.3. 
3. Heavy water: 
The plant must produce a maximum amount of D.sub.2 O or water enriched with 
deuterium. 
Solution of problem 
3000 m.sup.3 /year waste water on 375 kg/h waste water containing 200 
Ci/m.sup.3 T.sub.2 O and 150 ppm D/D+H are first mixed with 299,625 kg/h 
fresh water containing 150 ppm D/D+H, to form a flow of 30,000 kg/h water 
containing 0.25 Ci/m.sup.3 T.sub.2 O and 150 ppm D/D+H. 
This water is used as the feed (compare FIG. 1, reference 1 and FIG. 2, 
reference 1') for a plant for producing deuterium and tritium depleted 
water. 
The following Table gives numerical examples for three cases. Cases I and 
II correspond to the methods described above, in that the molar throughput 
of ammonia (see line "e") is more than two-thirds the molar throughput of 
water (see line "b"). Case III does not correspond to the previously-given 
conditions, in that the molar throughput of ammonia is less than 
two-thirds of the water throughput. 
______________________________________ 
CASE 
I II III 
______________________________________ 
Throughput in line 1 
(a) in kg/h H.sub.2 O 
300 000 300 000 300 000 
(b) in kmol/h H.sub.2 O 
16 666 16 666 16 666 
(c) in kmol/h H 33 332 33 332 33 332 
Throughput in line 40 or 42' 
(d) in kg/h NH.sub.3 
255 000 217 600 164 220 
(e) in kmol/h NH.sub.3 
15 000 12 800 9 660 
(f) in kmol/h H 45 000 38 400 28 980 
Temperature of 130 130 130 
columns 3 or 3' (.degree.C.) 
Pressure in column 3 or 
20 20 20 
3' (bars) 
NH.sub.3 recycled from 
column 7 or 7' 
(g) in kg/h NH.sub.3 
165 642 163 332.49 
160 036.042 
(h) in kmol/h H 29 230.96 
28 823.38 28 241.65 
H.sub.2 O stripping vapor from 
column 7 or 7' 
(i) in kg/h H.sub.2 O 
51 989.42 
47 081.54 40 076.589 
(j) in kmol/h H 
5 776.608 
5 231.282 4 452.954 
(k) Throughput of hydrogen 
atoms in the liquid in column 
3 or 3' kmol/h H 
[Sum of (c) + (h) + (j)] 
68 339.57 
67 386.6634 
66 026.609 
1. Throughput of hydrogen 
atoms in vapor rising in 
column 3 or 3', kmol/h 
[Sum of (f) + (h) + (j)] 
80 007.57 
72 454.6634 
67 674.609 
Separating factor for isotope 
exchange reaction 
NH.sub.3 + HDO .fwdarw. NH.sub.2 D+ H.sub. 2 O 
at 130.degree. C. 
0.996 0.996 0.996 
Separating factor for isotope 
exchange reaction 
NH.sub.3 + HTO .fwdarw. NH.sub.2 T+ H.sub.2 O 
at 130.degree. C. 
0.98 0.98 0.98 
Number of isotope 
20 20 50 
separating stages 
Assumed efficiency of 
100 100 100 
these stages (%) 
Deuterium concentration 
in ppm for D/D + H in: 
Line 1 or 1' 150 150 150 
Line 40 (FIG. 1) or 
8.79 5.207 1.2252 
42' (FIG. 2) 
At vapor inlet into column 
9.32 7.46 11.178 
3 or 3' 
At vapor outlet from 
112.49 126.7303 150.7476 
column 3 or 3' 
At liquid inlet into column 
130.785 138.2404 150.370307 
3 or 3' 
At liquid outlet from 
10 10 20 
column 3 or 3' 
Tritium concentration in 
in mCi/kmol H in 
Line 1 or 1' 2.25 2.25 2.25 
Line 40 or 42' 0.265 0.2228 0.045 
At vapor inlet column 
0.2672 0.245 0.1643 
3 or 3' 
at vapor outlet from 
1.7311 1.9415 2.32239 
column 3 or 3' 
At liquid inlet into column 
1.98394 2.0941 2.2858967 
3 or 3' 
At liquid outlet from 
0.27 0.27 0.27 
column 3 or 3' 
Tritium concentration in 
water, Ci/m.sup.3 
In line 1 or 1' 0.25 0.25 0.25 
In line 6 or 6' (Product) 
0.03 0.03 0.03 
Heavy water extraction, 
46.66 46.66 43.33 
corresponding to kg/h 
D.sub.2 O 
______________________________________ 
As the numerical example shows, the feed water can be depleted to 10 ppm in 
only twenty separation stages if the molar throughputs are chosen in 
accordance with the methods of the invention. (Cases I and II). 
The corresponding concentrations in line 40 (FIG. 1) or 42' (FIG. 2) are 
8.79 and 5.207 ppm, corresponding to a depletion factor of 12.8 (Case I) 
and 24.3 (Case II) in tower 14 or 14'. Case III, which does not correspond 
to the molar throughput requirements of the invention, cannot attain the 
proposed depletion to 10 ppm. 
Even to obtain 20 ppm, fifty separating stages instead of twenty are 
required, as shown by the numerical example. 
In case III, the concentration in line 40 (FIG. 1) or 42' (FIG. 2) is 
1.2252 ppm, corresponding to a depletion factor of 123. 
The same applies to tritium, as shown in the Table. 
The invention thus provides a relatively economic method of obtaining water 
free of deuterium and tritium as well as by-products enriched with 
deuterium and/or tritium or nitrogen enriched in the isotope .sup.15 N.