Comparison of methods for separating small quantities of hydrogen isotopes from an inert gas Page: 4 of 8
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" Free tritium gas is converted to the water form which
is >20,000 times more hazardous than the Q2 form;
" While the original stream has tritium in the Q2 form,
this is converted to water, only later to be reduced
back to the desired Q2 form. It would be preferable
to maintain the tritium as Q2;
" The multistep process is cumbersome, somewhat
complicated and occupies considerable space; and
" The magnesium bed process generates tritiated,
metal oxide waste comparable to the amount of wa-
ter processed. Consumption of magnesium beds is
not only expensive, but the resulting tritiated waste is
also an environmental hazard.
B. Technology Alternatives
The primary goal of this study was to identify processing
techniques which are simpler, avoid water formation and are
essentially waste free. Opportunities for improvements to the
present flush nitrogen processing system were considered
rather broadly. Seven technologies were identified which
could potentially form the basis for such an improved process.
These technologies are summarized in Table 1. In the next
section each technology will be described and evaluated.
I. TECHNOLOGY DESCRIPTIONS
A. Cryogenic Distillation
Description: Distillation separates materials based on differ-
ences in boiling points. Cryogenic distillation is required to
separate nitrogen and hydrogen isotopes because of their low
boiling points.
For this separation the bottoms product would be recovered at
about 96.5 K and the top product would be recovered at about
71.5 K. The top temperature was chosen to be above the
freezing point of pure nitrogen, which is approximately
63.2 K. Using these conditions the column top product would
be composed of 91.3% hydrogen isotopes and about 8.7%
nitrogen. The liquid nitrogen product (the column bottoms)
contains a mole fraction of hydrogen isotope of about 8x10-3.
This distillation column is operated at about 1300 torr. The
column employs fourteen theoretical trays. The column, in-
cluding a surge volume, will be two and one-half feet tall.
Evaluation: The advantages and disadvantages of this
method are:
Advantages
" Large boiling point differences resulting in relatively
easy separation
" Relatively small distillation column (2.5 feet)
Disadvantages
0 Startup/Shutdown of cryogenic equipmentTable 1
Nitrogen Flush Gas Processing Technologies Considered
Technology
Cryogenic Distillation
Oxydation-Adsorption-Palladium Membrane Reactor
Hollow Fiber Membrane
Getters
Permeators
Cryogenic Adsorption
CryoTrap
" Potential plugging especially in product stream which is
near N2 freezing point
" Top product requires further processing
The primary consideration for distillation is that the freezing
point of nitrogen is higher than the dew point of the hydrogen
isotopes at the desired top operating conditions. Therefore
the desired top product composition, essentially pure hydro-
gen isotopes, cannot be obtained. Additionally, it is some-
what difficult to work with cryogenic equipment. Thus, this
technology is not recommended for the stated application.
B. Oxydation-Adsorption-Palladium Membrane Reactor
Description: For the "base case" process the tritium is oxi-
dized and collected as water. Subsequently the tritium is re-
covered by reducing the water in the presence of Mg. This
latter process produces substantial tritium-contaminated mag-
nesium oxide waste. Alternatively this water could be proc-
essed with an essentially waste-free Palladium Membrane
Reactor (PMR). This device uses the water-gas shift reaction,
Q2O+CO++Q2+CO2, in concert with a Pd/Ag membrane to
recover hydrogen isotopes from water. The performance of
PMR technology has been demonstrated at LANL over the
last several years. This technology is very similar to per-
meator technology, with the exception that catalyst is loaded
in the high-pressure side of the permeator and gas addition
(CO) is required. The system is similar to the two-stage per-
meator design described elsewhere in this paper, with the ex-
ception that the permeators have a catalyst to promote the
reaction of Q20 and CO. The pumping requirements are the
same.
Evaluation: The advantages and disadvantages of this sys-
tem are:
Advantages
* Provides waste-free water processing system
Disadvantages
" Unnecessarily oxidizes tritium to water which is harder to
transport (condenses) and is much more hazardous
" Multistep process
The two-stage PMR system would appear to be well suited for
the Q2/N2 separation application if the Q2 were already con-
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Willms, R.S.; Tuggle, D.; Birdsell, S.; Parkinson, J.; Price, B. & Lohmeir, D. Comparison of methods for separating small quantities of hydrogen isotopes from an inert gas, article, March 1, 1998; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc711226/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.