Means for separating isotopes of hydrogen based on the principle of gas chromatography

The invention describes a means for separating isotopes of hydrogen which is based on the principle of gas chromatography and is suitable for separating large amounts of hydrogen as is necessary for fusion experiments. The substantially higher throughput in comparison to that of other analysis devices based on gas chromatography is rendered feasible by the fact that the packings of the separation columns and the collectors for absorbing the separate species contain inert additives which ensure even distribution of the gas flow.

BACKGROUND OF THE INVENTION 
This invention concerns a means for separating isotopes of hydrogen 
contained in a gas mixture based on the principle of gas chromatography, 
comprising a flow passage for a carrier gas containing a feed pump, at 
least one separation column with a packing, including a fine-grained 
active separation material, at least one collector which is controlled so 
as to be switched on by a valve arrangement, with a packing of a 
hydrogen-absorbing material, and a valve arrangement for controlling the 
separation process. 
Tokamak fusion experiments on the scale of TFTR or JET will have a daily 
throughput in tritium operation amounting to some thousand Curies (Ci) of 
tritium (T.sub.2) serving as a reactor fuel. Only a small amount of this 
tritium is "burned"; the largest portion (above 90%) can be used again. 
However, before doing so it is necessary to clean this reactor fuel and to 
separate the light hydrogen and its compounds, i.e. H.sub.2, HD and HT, 
and to recover the isotopes D.sub.2 and T.sub.2 and the compound DT in a 
pure state and in large amounts (a few standard liters per day). In other 
fields of technology the need arises to recover small amounts of a certain 
isotope of hydrogen from a surplus of another isotope of hydrogen or 
another gas. 
There are various processes for separating isotopes of hydrogen. In 
practical operation, low-temperature distillation is suitable only for 
separating large amounts of hydrogen isotopes; the throughput of expedient 
units is 10.sup.3 times higher than that required for the above-mentioned 
experiments and the dead stock in the cryogenic colunns is above 10.sup.5 
Ci. Diffusion processes require high expenditure because the separating 
factor per stage is only about 2. Bipolar electrolysis with a separation 
factor between about 4 and 8 is still in the development stage. 
Gas chromatography for separating isotopes of hydrogen has heretofore only 
been used for analytical purposes. All isotopes of hydrogen and their 
compounds can be quantitively separated by means of suitable prior art 
analysis devices based on gas chromatography. However, the throughput is 
only in the region of a few microliters and reaches a maximum of about 
one-tenth of a milliliter. It is already known (from "FUSION TECHNOLOGY", 
1980, Pergamon Press Oxford and New York, pp. 571-577 and "ACHEMA-82-20. 
Ausstellungstagung fur chemisches Apparatewesen" from 6 to 12 June 1982 in 
Frankfurt) to use gas chromatography on a larger scale for separating 
isotopes of hydrogen. 
BRIEF DESCRIPTION OF THE PRIOR ART 
British patent specification No. 825,934 describes a method for separating 
deuterium (D.sub.2) from a mixture of H.sub.2, HD and D.sub.2 which has 
come to be known as the "displacement method". The hydrogen isotopes are 
separated isotopically in palladium which is contained in a separation 
column as a powder mixed with asbestos fibres at a weight ratio of 10:3. 
The asbestos serves as an inert carrier for the metal powder. The 
palladium catalyses the conversion of the molecule HD into the molecules 
H.sub.2 and D.sub.2 during constant flushing with H.sub.2. Thus it does 
not relate to the separation of H.sub.2, HD and D.sub.2 but rather to the 
recovery of D.sub.2 from the gas mixture; the molecule HD disappears 
completely and the originally present H.sub.2 mixes with the surplus of 
the H.sub.2 flush gas. The displacement method can therefore only be used 
for enriching a component of a mixture but not for quantative separation 
of the initial mixture. A further serious drawback of this prior art is 
that the displacement gas H.sub.2 has to be desorbed again after each 
experiment before the separation column can be used again. 
French Pat. No. 1,478,542 describes a process for separating isotopes of 
hydrogen by reverse flow enrichment. This process operates on the basis of 
moved adsorption layers and a displacement gas (e.g. N.sub.2). 
U.S. Pat. No. 2,863,526 is a fractionating enrichment process for 
separating hydrogen isotopes in which the hydrogen isotopes are absorbed 
at room temperature in palladium and then desorbed again at an increased 
temperature. Palladium beds are used for absorption. In the palladium beds 
the palladium is mixed with quartz sand. The serious problem of so-called 
peak uncertainty associated with chromatographic processes does not occur 
in this process. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a means for separating 
hydrogen isotopes and their compounds based on the principle of gas 
chromatography, which enables the separation of large sample charges with 
a high separation factor and which is capable of storing the separated 
isotopes and their compounds. 
A more even throughput across the whole cross section of the separation 
column is achieved and the formation of a passage is avoided by the fact 
that the separation column contains inactive fillers in addition to the 
active substance, preferably aluminium oxide. About 500 milliliters of a 
mixture of hydrogen isotopes can be separated per charge and a substantial 
(above 99.9%) separation of D.sub.2, DT and T.sub.2 is ensured with a 
weekly throughput of about 25 standard liters or about 10.sup.4 Ci of 
T.sub.2. Thus a simple method is provided for quantative re-use of the 
"fusion fuel" DT and T.sub.2. By means of special features of the 
absorption means, in particular the use of relatively coarse fillers 
comprising a material of sufficient heat conductivity, e.g. stainless 
steel, substantial absorption of desired separated species of isotopes can 
be achieved from a plurality of subsequent separation processes. 
The means of the invention may be used generally for cleaning, separating, 
enriching and storing hydrogen isotopes, e.g. for recovering hydrogen 
isotopes mixed in another gas, e.g. an inert gas, or for recovering small 
amounts of another hydrogen isotope.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment example of a preferred means for separating hydrogen isotopes 
and their compounds based on the principle of gas chromatography is shown 
in FIG. 1. For the sake of simplicity, FIG. 1a shows a portion of this 
means which is active in the separation process. The illustrated means 
contains a circulation system for a carrier gas, in particular helium. The 
carrier gas circulation system contains a diaphragm-type compressor MP, 
upstream of which a buffer volume AB1 is arranged and downstream of which 
a second buffer volume AB2 is arranged. The buffer volumes serve to smooth 
pressure knocks which might otherwise occur when introducing the gas 
mixture to be separated and when reversing valves. 
The carrier gas circulation systems branch off from the outlet of the 
buffer volume AB2 into two optionally switchable branches, each of which 
contains a flow meter Q1 and Q2 respectively, a sample volume PV1 and PV2 
respectively, a separation column K1 and K2 respectively and a hydrogen 
isotope detector D1 and D2 respectively and also a number of valves and 
bridge connections, the arrangement of which can be seen in FIGS. 1 and 1a 
and the function of which will be explained hereinafter in greater detail. 
The carrier gas circulation system continues from the outlet side of the 
detectors D1 and D2 respectively via a further valve arrangement on to one 
of three collectors A1, A2 and A3 which are optionally switchable. The 
circulation system continues from the outlet of the switched collector via 
a further hydrogen isotope detector D3 and a further flow meter Q3 to the 
inlet side of the buffer volume AB1. 
Pressure gauges P are arranged at various points as shown in the drawing. 
Four gas containers TG, which contain helium, H.sub.2, D.sub.2 or HD and 
which communicate with the means in the manner as shown by way of pressure 
reducing valves and check valves, are provided for supplying carrier gas 
to the means and for filling the sample volumes PV with a test gas 
mixture. A further pump TP is provided for evacuating the means and for 
pumping off the separated isotopes. 
The tritium-proof double membrane compressor MP constantly pumps the 
carrier gas flow at a throughput of 10 l/min, for example. A given amount 
of the gas mixture to be separated, which may be of optional composition 
as regards the hydrogen isotopes, is injected in a single "pulse", i.e. as 
a compact gas stopper, via the calibrated sample volumes PV1 or PV2 into 
the circulating carrier gas which preferably comprises highly pure 
(approx. 99.999% pure) helium. This pulse or gas stopper then passes from 
the sample volumes PV1 or PV2 with the carrier gas flow through the 
passage (heavily lined in the drawing) into the separation columns K1 or 
K2. Each separation column comprises four subsections which are connected 
in series and each of which has a cleading 12 made of copper piping with a 
wall thickness of 2 mm, an internal diameter of 50 mm and a length of 750 
mm. Thus each separation column K has an overall length of 3000 mm. The 
filling in the separation columns comprises a pre-treated Al.sub.2 O.sub.3 
powder 14 and each column is arranged in a cryostatic temperature 
regulator KS1 or KS2 which is filled with liquid nitrogen. The gas 
pressure in the separation column is preferably above atmospheric pressure 
and may, by way of example, amount to between 2 and 5 bars, preferably 3 
bars. 
The separation of the individual hydrogen isotopes and hydrogen isotope 
compounds now takes place in the switched on column K1 or K2 because of 
succeeding adsorption and desorption processes occurring at the boundary 
layer between the aluminium oxide filling and the carrier gas flow. 
After leaving the column the separated species are registered by means of 
the gas-tight or tritium-tight helium ionisation detector D1 or D2. One or 
other, or all of the isotopes or isotope compounds can now be "cut out" 
individually from the carrier gas flow, depending on the specific 
requirements. The collectors A1 to A3 used for this purpose are preferably 
made of stainless steel and are filled with a reactive metal or a reactive 
metal alloy which is capable of absorbing hydrogen in sufficient amounts 
and sufficiently quickly. The collector provided for absorbing a given 
isotope is switched on by activating the appropriate valves in the carrier 
gas flow when the desired isotope or the desired compound leaves the 
separation column. The valves may be automatically operated by the 
detector signal or in accordance with a separation time which is 
characteristic of and constant for each isotope and each compound. 
The passage of flow of the carrier gas containing the separated species 
from the column K1 or K2 to the valves, which allow the carrier gas to be 
discharged to a desired collector, is indicated by a broken line in FIG. 
1. In the means according to FIG. 1 three collectors A1, A2, A3 are shown 
which may be opened by means of the illustrated valve arrangement to 
optionally communicate with one of the carrier gas currents. 
The further detector D3 arranged behind the respective open collector is 
for monitoring purposes and will no longer record the isotope absorbed by 
the collector if the means is functioning properly. From detector D3 the 
carrier gas flows via AB1 back to the membrane compressor MP. 
Since the sample volume is larger by the factor 10.sup.4 than in the case 
of gas chromatography separation processes, particular attention must be 
devoted to the process parameters which directly influence the sample 
amount. These are the following: 
The cross section of the column: the sample volume can be increased with 
increasing cross section of the stationary phase, i.e. the cross section 
of the separation column. At the same time the separation efficiency is 
affected, however. Circular cross sections with a diameter of about 40 to 
60 mm, in particular 50 mm, are recommended. 
The length of the column: the separation efficiency increases with 
increasing length of the column, but the separation time also increases. 
The total length of the column may amount to between 2000 and 5000 mm; 
3000 mm has proved to be an expedient length. 
The carrier gas velocity: an increase in the carrier gas velocity shortens 
the separating time, but also the separation efficiency. A value in the 
magnitude of 10 l/m has turned out to be satisfactory in the embodiment 
example described. 
The separation material: the active separation material, i.e. the 
stationary phase in the separation column, which may comprise special 
molecular sieves (zeolites), porous polymers, activated carbon or complex 
polymer molecular sieves and special pre-treated aluminium oxides, has a 
decisive influence both on the sample volume and on the separation 
efficiency. A preferred material is ordinary commercial gamma-Al.sub.2 
O.sub.3 which has been treated in a special manner and which has 
preferably a granulation value of about 60 to 210 microns. The grain 
distribution is in keeping with the normal distribution with a maximum 
(70%) at approximately 140 microns. 
The separation activity of the aluminium oxide, which can be described in 
terms of its polarity depends on the water content and can be seen in a 
chromatogram in the form of peaks. If the separation substance is 
completely free of water, the retention period of the individual species 
increases sharply, which means that the column is too active and retains 
the molecules to too great an extent, the result being an undesirably long 
separation period for the charges. 
In order to effectively prevent lateral diffusion in the filling, 
enrichment or "nests" caused by cavities, concentration shifts due to 
various packing densities and similar adverse affects, the filling has to 
be packed evenly, both over the cross section of the column and over the 
length of the column. If it is packed too tightly, the result will be too 
high a carrier gas pressure which will give rise to longer separation 
periods. 
These problems are solved by adding about 0.5 to 1 percent by weight of 
SiO.sub.2 fibres 16, which preferably have a mean diameter of 0.1 mm and a 
mean length of 2 mm. This addition renders it possible to distribute the 
aluminium oxide evenly in the packing using medium pressure. In addition, 
the activity of the aluminium oxide is optimized by having the water 
content of the aluminium oxide at about 1 percent by weight. This can be 
done in the described embodiment example by heating the separation column 
for 72 hours at 410.degree. C. in a flow of helium (approximately 50 
l/min). The above-mentioned disturbances can be thus avoided and the 
sample volume can be substantially increased. 
The temperature of the separation material: to separate hydrogen isotopes 
it is generally necessary to cool the separation column, e.g. to the 
temperature of liquid nitrogen (approx. 77K). It must be ensured that the 
temperature over the column cross section is as even as possible; in 
particular the heat added along with the carrier gas current must be 
discharged quickly through the walls of the column. For this reason, a 
wall material which affords high heat conductivity, in particular copper, 
is used. 
The detectors: ionisation detectors are used for monitoring the separated 
isotopes. The ordinary commercial He ionisation detectors are, however, 
neither high vacuum-tight nor diffusion-tight. The detector has therefore 
been arranged in a tritium-proof stainless steel housing with ultra-high 
vacuum-tight high-tension feed-throughs. 
Collectors: it is known that, for example, uranium in its pyrophorous state 
or the alloy Zr/Al when cold can absorb large amounts of hydrogen, e.g. in 
the case of uranium when the compound UH.sub.3 is formed. The process of 
binding hydrogen by metals or metal alloys has heretofore only been 
applied in stationary operation, i.e. the hydrogen is passed over the 
respective metal and one waits until the amount of hydrogen is bound which 
corresponds to the temperature and pressure applied. Such a process is, 
however, not suitable for the described separation means since the 
hydrogen isotopes have to be removed as completely as possible from the 
flow of helium at a pressure of up to 3 bars and a carrier gas velocity of 
approximately 10 l/min. With the volume of the container amounting to 0.5 
liters, the above-mentioned flow velocity would result in a twenty-fold 
gas exchange per minute. 
In a preferred embodiment of the collectors A1 to A3 600 g of pyrophorous 
uranium powder 20 were loosely arranged in three levels over frits made of 
stainless steel which are closely welded to the housing. An enlarged 
schematic view of this embodiment is shown in FIG. 2 in the case of a frit 
22. Due to the relatively high flow velocity of the carrier gas containing 
the hydrogen isotopes in the uranium powder, due to local overheating 
during hydrogen absorption and due to partial sintering of the powdered 
uranium, there is a danger that a passage might be formed in the uranium 
powder mass. To avoid this, balls 24 made of stainless steel and having a 
diameter of 2.5 mm, by way of example, are added to the uranium powder 20. 
These balls ensure heat discharge to the wall of the vessel 30 comprising 
a good heat-conducting material and also counteract sintering. Helical or 
spiral bimetallic springs have also proved successful instead of the balls 
made of stainless steel or in addition to them, as inert additives in the 
uranium powder packing, because they loosen the uranium powder when 
temperature changes occur by virtue of their unrolling and rolling up 
again and they also avoid baking and the formation of a passage. 
The tight welding of the frits to the wall of the collector and similar 
tight welding of the gas inlet and outlet pipes and the container ensure 
that gaps, through which losses could occur due to the high carrier gas 
velocity, can be effectively avoided. 
A further important object of the collector having for example pyrophorous 
uranium as a storage metal is to collect a special isotope from as many as 
possible of the separation cycles. If the described separation means is 
used for example for a fusion experiment, the tritium should be separated 
from the used fuel of as many individual experiments as possible and then 
collected. A week seems a practical period for an experiment to last, so 
that some 10 liters of storage capacity are necessary. 
The theoretical sorption capacity of 600 g of uranium amounts to about 
25.times.10.sup.4 Ci of tritium. In the sorption process from the quickly 
flowing helium carrier gas the isotope is first built up in the outer 
layers of the agglomerates of uranium particles which become quickly 
saturated. It must therefore be ensured that such loading of the outer 
layers is transferred to the inside, i.e. that as homogenous a "mass" 
loading as possible of the total reactionable metal occurs. This 
transposition is achieved by applying temperature gradients, i.e. the 
border layers are heated for a short time while the inside of the uranium 
agglomerate remains cool, and the hydrogen isotopes will shift away from 
the boundary zones. The embodiment of this temperature gradient is 
supported by the interpolated additives, in particular the stainless steel 
balls. When the vessel wall 30 is heated for a short time to approximately 
between 270.degree. and 275.degree. C. by means of an electrically 
operable heating jacket 32 disposed on the outside, the carrier gas 
comprising the conductive helium is heated and it transmits at least some 
of its heat to the stainless steel balls. The latter transmit the heat to 
the adjacent loose uranium powder and thus cause a hydrogen release 
appropriate to the temperature. Since pyrophorous uranium is a poor 
conductor of heat the inner part of the agglomerate remains cooler so that 
the respective hydrogen isotope is absorbed in accordance with the 
hydrogen dissociation pressure. An enlarged schematic view of this is 
shown in the circle in FIG. 2. In this way, the hydrogen loading of the 
sorption metal is evenly distributed and the reception capacity of the 
collector is enhanced. 
Using such a collector and the above-described packing it is possible to 
attain complete absorption of up to several hundred milliliters of 
hydrogen isotopes from the stream of gas. 
The described embodiment example may of course be varied in several ways. 
The stainless steel ball may have a diameter of between 2 and 3 
millimeters or may have a diameter outside this range, and a different 
volume ratio to 1:1 of the active absorbing metal powder and the stainless 
steel balls or other additives may also be used, e.g. 3:1 and more; 
generally the filler bodies should be separated by active material. 
Other materials may be used in the separation column instead of the 
gamma-aluminium oxide mentioned above. When using gamma-aluminium oxide 
the water content is preferably 1 percent by weight; it may, however 
deviate from this value and be, for example, between 0.5 and 2.5 percent 
by weight. The added fiber or needle-shaped silicon dioxide members may 
also have other lengths and diameters, e.g. length between 1 and 3 mm and 
mean diameter between 0.5 and 0.3 mm, and the share of these fillers can 
also be increased to up to 5 percent by weight of the aluminium oxide. The 
values and parameters stated in the description of the embodiment example 
are, however, preferred. 
When operating the means illustrated in FIG. 1 a reversal of the valves may 
be achieved by means of a programmed control system (not shown) in 
conjunction with a time control or the detector output signals for the 
selection of the individual separated isotopes. 
When the means is put into operation it is first evacuated and heated and 
then flushed with helium. Then the membrane compressor MP is put into 
operation, the helium carrier gas is pumped around and the operating 
parameters provided for separation, e.g. the temperature of the column, 
are set. 
During this preparation the valves V101, V104, V105, V107 and V111 are open 
and all other valves are closed so that the carrier gas is pumped from the 
membrane compressor MP through the column K1 and the detectors D1 and D3. 
The sample volumes PV1 can be evacuated by means of the pump TP through 
the pipes with the open valves V301, V302 and V303, while the valves V304 
and V305 are closed. Then the valve V301 is closed and the isotope mixture 
to be separated is fed in from a supply which is formed in the embodiment 
example according to FIG. 1 by the container TG contained in the rectangle 
shown by means of a broken line. The the valves V302 and V305 are closed 
and the means is then ready for the separation process. 
For starting a separation process the valve V101 is closed and at the same 
time the valves V102 and V103 are opened so that the carrier gas removes 
with it the isotope mixture contained in the sample volume PV1, which is 
to be separated, and places it as a compact pulse or stopper in the 
separation column K1. After a certain time, which is constant from charge 
to charge under prescribed operational conditions and apparatus 
parameters, the first hydrogen isotope appears at the outlet of the column 
K1 which is reflected by the detector D1. Then the collector provided for 
sorption and collection of the respective isotope, e.g. collector A1, is 
communicated to the carrier gas flow. This may be accomplished either by a 
time-independent control or by means of the output signal of the detector 
D1 in a manner which is not described in more detail. For communicating 
the collector A1, the valve V111 is closed V110 and V11 and V12 are opened 
so that the gas will now flow through the collector A1 to detector D3 
where the respective isotope is then absorbed and removed from the carrier 
gas. After a certain time or upon control by the output signal of the 
detector D1 the gas flow is then switched over to the next collector A2 so 
as to remove and collect the next isotope or isotope mixture from the 
carrier gas. For this purpose valves V110, V11 and V12 are closed and 
valve V108 and valves V21 and V22 are opened so that the gas can now flow 
through the collector A2. 
In the same way the collector A3 can be communicated via valve V109 and 
valves V31 and V32. If more than three isotopes or isotope mixtures are to 
be removed an appropriate number of collectors and valve arrangements must 
be provided. 
While separation is being carried out by means of column K1, the sample 
volume PV2 can be filled in an analogous way for preparing a subsequent 
separation process using column K2. The next charge can then be separated 
in the same way by means of column K2 while using the valve arrangement 
associated therewith, which will become immediately clear from the above 
description. The active separation material is generally a non-metallic 
particulate material of no essential catalytic activity.