Note: Descriptions are shown in the official language in which they were submitted.
~459~7
This invention relates to an improved system
for the separation from waste gas of radioactive krypton
and xenon. Such waste gases are derived, for example,
from the reprocessing of nuclear fuels or from nuclear
reactors, wherein there are produced the radioactive
isotopes of Kr and Xe. It is common for such waste
gases to contain, in addition to other components,
nitrogen and argon as the carrier gases, and for the
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waste gas, after the other components have been separated,
to be cooled and thereafter freed o krypton by
distillation.
An example of a conventional process is
described in "A Cryogenic Approach to Fuel Reprocessing
Gaseous Radwaste Treatment" (J.S. Davis, J.R. Martin,
Paper for Presentation at The Noble Gas Symposium,
Las Vegas, 1973). This process comprises first subjecting ;
a waste gas from nuclear fuel reprocessing plant
consisting essentially of nitrogen, oxygen, oxides of
nitrogen, carbon dioxide, and water, as well as argon,
krypton, and xenon, to a pretreatment in order to remove
the oxygen, nitrogen oxides, carbon dioxide and water.
In this process, the oxygen and the nitrogen oxides are -~
catalytically reduced to nitrogen and water by the
introduction o hydrogen. The resultant water and the
carbon dioxide are removed by adsorption. The carrier gas
of N2 + Ar thus freed of all other components except for
the kryp-ton and xenon impurities, is thereafter fed to a
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low-temperature separation plant wherein in a first
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rectification column a fr~ction containing krypton and
xenon is recovered. Then, in a second rectification
column, krypton and xenon ~re separated from each other.
It is thus seen that in the conventional process, wherein
a highly concentrated radioactive krypton fraction is
treated, two rectification column are required, ~ogether
with the associated conduits and switching members.
It is also to be noted, however, that the
radioactivity of the xenon may be substantially
diminished before the aforedescribed treatment of the waste
gas because its half-life is only 5.3 days (xenon-133),
and there is the possibility of conducting the waste gases
through "holdup" zones prior to treatment. In contrast
thereto, the radioactivity of krypton-85 (half life: 10.
years) remains prac-tically constant during the time
generally available for the waste gas treatment.
An object of one aspect of this invention is
to provide an improved system yenerally of the above-
mentioned`type, and especially wherein there is a significant
savings in the expenditure required for the low-temperature
section.
According to the broad aspect of this invention,
the improved system is obtained by separating the xenon hy
adsorption even before the rectification step. ~ctivated carbon, - ;
for example, can serve as the adsorbent. General information
concerning adsorption processes and techniques is contained in
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S967
J.W. carter "~dsor~tion Processes" (Chemical and Process
Engineering, Sept. 1966, pp. 70-77) and ~.A. Johns-ton
"Designing Fixed-Bed Adsorption Columnsl' (Chemical
Engineering, Nov. 1972, ~p. 87-92).
By virtue of this pre-adsorption step, only
a single rectification column is required, and thus the
protective measures to be taken against the radioactive
radiation emanating from the highly concentrated krypton
fraction in the sump of thi~ column can be restricted to
only the necessary and unavoidable minimum.
Since, during the adsorption of the xenon, ~-
traces of krypton are concomitantly adsorbed, it is
advantageous, according to another aspect of the invention,
to provide that the regeneration of the adsorbers is
conducted in at least two purging phases, wherein in the
first phase primarily krypton is desorbed and the
krypton-containing purge gas is recylced into the process,
while in the second phase the xenon is desorbed and the
xenon-containing purge gas is removed from the process.
.
In accordance with another aspect of this invention, '
a carrier gas mixture freed at least partially of the
krypton is utilized as the purge gas. The purge gas
withdrawn ~rom the low-temperature plant and comprised
almost exclusively of nitrogen and argon is divided into
two portions, one portion absorbing the adsorbed krypton
and the other portion absorbing the xenon. Since the
krypton, despite its low quantity, has a high radioactivity
as compared to the xenon, the krypton-containing purge
gas is not discharged from the process, but rather recycled
to the process at a suitable location, e.g., the
low pressure side of the compressor. ~n contrast thereto,
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the xenon-containing purge gas can eikher be discharged
to the outside atmosphere or it can be further processed
to obtain its components. If desired, the radioactivity
of the xenon can be reduced further in holdup zones
located either upstream or downstream of the adsorption
zone.
According to another aspect of the process
according to this invention, the xenon, optionally
together with one of the other components contained in
the waste gas, e.g., carbon dioxide, is separated in a
mixed-bed adsorber or in divided adsorber beds. In ;
this connection the combined xenon-carbon dioxide
adsorption can be conducted in two series-connected adsorber
beds; in the second purge phase, xenon and carbon dioxide
are distributed separately over the beds; and in a third
purge phase with parallel-connected purge streams, xenon
and carbon dioxide are purged out separately. The
adsorption phase is terminated as soon as the xenon
adsorption front has traveled through the second adsorber
bed to an extent of about 75%. The relative dimensions
of the two adsorber beds are designed so that at this
instant the carbon dioxide front is still in the interior
of the first adsorber bed. During the first purge phase,
the xenon front advances to the end of the second adsorber
bed. During the second purge phase, owing to the ready `
desorbability oE the xenon as compared to the carbon
dioxide, the xenon is almost completely desorbed from the
first adsorber bed and is deposited in the second bed,
the carbon dioxide front advancing during this step to `
the end of the first bed. During the subsequent third
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purge ph~se, both beds are connected in parallel and
freed separately of xenon arld carbon dioxide. The
adsorber beds can contain zeolite and activated carbon,
for example.
As mentioned above, the waste gases from
nuclear fuel reprocessing plants or nuclear reactors
generally contain, as additional components, oxygen
and nitrogen oxides;-these are usually catalytically reduced -
to nitrogen and water with the use of hydrogen. Most of
the thus-formed water is removed in a separator. In
such a case, it is advantageous, according to another
aspect of this invention, to separate the residual water
in reversible adsorbers even prior to the adsorptive
xenon removal from the waste gases. Such adsorbers
are then purged in the regenerating phase with hydrogen,
and the H2O-~I2 resultant mixture is fed to the
catalytic reduction stage. By the introduction of the
H2O-containing hydrogen into the reduction stage,
there is the advantage that even the residual water
absorbed in the adsorbers is finally removed in the separator.
Suitable exemplary adsorbents for the water include, but
are not limited to, silica gel and zeolite.
The drawing is a schematic view of the preferred
embodiment of the invention.
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A radioactive gaseous waste stream ls introduced
via conduit 1, comprising 9000 mol/h of 67 mol-~ nitrogen,
16 mol-~ oxygen, 5 mol-~ water, 8 mol-% nitrogen dioxide,
3 mol-% nitrogen monoxide, and 1 mol-% argon, as well as
800 mol-p.p.m. xenon and 160 mol-p.p.m. krypton and
containing traces of N2O, CO2 and CO. The radioactivity
caused by the radioactive krypton isotope krypton-85
is 0.62 Ci/s. In a compressor 2, this waste gas is
compressed from 1 bar to about 6 bars and introduced
into a catalytic reduction stage 3 together with a
hydrogen-enriched gas fed via conduit 24, as well as a
recycling gas fed via conduit 25 tthis recycling gas will
be defined in greater detail below). In this ca-talytic
reduction stage, the nitrogen oxides along with the
oxygen are reduced to nitrogen and water by reaction with
the hydrogen. Platinum can be utilized as the catalyst,
for example. In the subsequent phase separator 4, the
largest portion of the water is separated in the liquid
phase. To avoid the development of flames in the
reduction stage, the gaseous mixture to be conducted over
the catalyst must be diluted to such an extent that the
total content of oxygen plus nitrogen oxides is only at
most 3 mol-%. For this purpose, a corresponding portion
of the gas discharged from the separator is recycled by
means of a blower 5 to the inlet of the reduction stage.
The remaining portion of the gas withdrawn from the
separator wherein the proportions of oxygen and nitrogen
oxides are below 1 mol-p.p.m. and below 5 mol-p.p.m.,
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respectively, is fed to one oE the two silica gel
adsorbers 6 and 7, respectively, in order to remove
the residual water. These adsorbers can be periodically
switched over between the adsorption phase and the
regenerating phase. The purge of the adsorbers during
the regenerating phase is effected with hydrogen
introduced at 8 into the process. The H2O-containing
hydrogen is conducted to the reduction stage 3 via
conduit 24.
The thus-pretreated waste gas, at this point
- consisting essentially merely of the carrier gases
nitrogen and argon, as well as carbon dioxide, krypton
ans xenon, is now introduced into one of the three
periodically reversible adsorbers 9, lO or 11. In
this embodiment, the adsorbers are equipped with
zeolite and activated-carbon mixed beds wherein the
xenon as well as the carbon dioxide is adsorbed. In
this step, activated carbon is used for the xenon
adsorption, and zeolite having a pore size of 9 Angstrom ;
is used for the carbon dioxide adsorption. In addition
to xenon and carbon dioxide, krypton is concomitantly
adsorbed in minor quantities. The adsorption eront of
the krypton travels the most rapidly through the
adsorber, while the carbon dioxide front is the slowest.
As soon as the xenon front has migrated to about three
quarters through the adsorber, the switchover to the
regeneration phases is effected.
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In the ~irst regenerating phase, krypton is
desorbed at ambient temperature, and in the second
re~enerating phase carbon dioxide and xenon are desorbed
at about 420 K. The puriEied carrier gas mixture
withdrawn in conduit 18 from the low-temperature plant 13
is utilized as the purge gas. Since krypton is more
readily desorbable as compared to xenon or carbon dioxide,
the purge gas is divided into two unequal parts upstream
oE the adsorbers, and the smaller, e.g. about 10 to 30 ~
of the two partial quantities is conducted into the appropriate ~;
adsorber for krypton desorption. During the krypton
desorption, the adsorption front of the xenon is advanced to
the end oE the adsorber, since the purge gas flows through
the adsorber in the same direction as the waste gas did`
previously~ The krypton-containing purge gas is recycled
to the compressor 2 via conduit 19. The purge gas proportion
coming from the second regeneration phase and containing
xenon and carbon dioxide leaves the plant via conduit 20 `~
at 21. `
The krypton-containing carrier gas mixture,
almost completely freed of xenon and all other components,
is cooled to about 95 K in heat exchanger 12 and introduced
into the rectification column 13 under a pressure oE about
5 bars. The amounts thus introduced are, in detail:
7,760 mol/h nitrogen; 144 mol/h hydrogen; 108 mol!h argon,
and 1.44 ~ol/h krypton. The reflux liquid required for
the rectification in the rectification column 13 is produced
in condenser 17 by the indirect supply of cold with liquid
nitrogen from the storage tank 16. A liquid fraction enriched
in krypton to about 80% is withdrawn from the sump of the
separating column, entirely vaporized in evaporator 14 and
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heated to ambient temperature, withdrawn from the plant
at 15, and thereafter stored in metered quantities. The
withdrawal of the krypton fraction is preferably not
executed continuously, but rather once a day in a
quantity of respectively about 1.5 liters.
The purified carrier gas discharged from the head ~
of the separating column via conduit 18 contains at this ~-
point merely 0.1 mol-p.p.m. of krypton. This corresponds
to a decontamination factor of 1 : 1,800. The thus-
10 purified carrier gas is utilized, as described above, ~;
for purging the xenon carbon dioxide adsorbers 9, 10 and
11 .
Vaporized nitrogen is passed from column 13 via
conduit 22 through heat exchanger 12 where it is warmed,
and then withdrawn from the plant via condui~ 23.
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