Influence of Pressure and Oxygen Concentration on Nitrogen Isotope
Separation in N2-O2 DC Discharge
Nga Thi Anh Nguyen, Shinsuke Mori, Masaaki Suzuki
Department of Chemical Engineering, Tokyo Institute of Technology
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Abstract: Nitrogen isotope separation using plasma chemical reactions in N2-O2
electrical discharges system is performed in this study. The nitrogen isotope
separation process has been investigated in the influence of oxygen concentration
with a wide range of discharge pressure. The conversion of nitrogen increases
with an increase in oxygen concentration. In atmospheric pressure, the lower
oxygen concentration shows the higher nitrogen isotope separation factors in
compensation for very small conversion of nitrogen. The highest nitrogen
separation factor of 1.8 has been obtained at 0.03% of oxygen concentration in
discharge of the N2-O2 mixture at atmospheric pressure.
Keywords: nitrogen isotope, isotope separation, pressure, oxygen concentration
1. Introduction
The enriched isotope of 15N has been used in various
fields, such as agriculture, organic chemistry, life
science, etc., as a non-radioactive tracer. Recently,
greater attention has been given to the use of 15N in
the nitride fuels of the fast breeder reactors (FBRs).
Nitride fuels such as UN and PuN are known to have
many desirable properties such as high melting
point, high thermal conductivity and high breeding
ratio. Since the (n, p) reaction of the abundant
isotope 14N in natural nitrogen with neutron
generates the radioactive 14C, the presence of isotope
of 15N to avoid producing 14C elements in nitride
fuels is a necessary replacement for environmental
treatment. Therefore, enrichment of 15N is an
important process for the implementation of nitride
fuels.
So far, many developments in nitrogen isotope
separation processes have occurred. From the
beginning, conventional methods have been
conducted for the enrichment of the heavier nitrogen
isotopes. These studies were the cryogenic
distillation of NO with the separation factor of 1.027
or the ion exchange method in NH3-(NH4)2SO4
system with the separation factor of 1.021. However,
these methods have some disadvantages in high
energy consumption, multistage separation and
environmental risk as well. For investigating
alternative methods, in this study, we applied the
plasma chemical reaction in N2-O2 electrical
discharge for the nitrogen isotope separation
process.
In N2-O2 electrical discharges plasma system,
vibrational excitation of the first few vibrational
levels of a diatomic nitrogen molecule is operated by
free electrons. With the collision of two
vibrationally excited molecules, the excitation of a
higher vibrational energy molecule having close
vibrational level spacing is a faster process than that
of the lower vibrational energy molecule having
distant vibrational level spacing [1]. In addition, the
isotopic effect reported by Belenov et al. [2] leads to
the overpopulation of vibrational levels of a heavy
component because of its closer spacing.
14
N 2 ( v ) + 14 N 15 N ( w ) →
14
N 2 (v − 1) + 14 N 15 N ( w + 1), w ≥ v
(1)
Therefore, the enrichment of 15N in the nitric oxide
compound is achieved due to the difference in the
rates of the chemical reactions of the molecules
14 14
N N and 14N15N (excited in vibrational levels
ν ≥ 12) with O atoms [3].
N 2 ( v ≥ 12 ) + O → NO + N
(2)
The presence of oxygen and its concentration play
important role in the reactions of (1) and (2). In
1995, Gordiets et al. [4] published a calculation
about the effect of oxygen percentages on the
nitrogen vibrational distribution at low pressure of
N2-O2 glow discharge. The results demonstrated that
the increase in oxygen percentage leads to the
reduction of nitrogen vibrational distribution. In
2000, for another research of discharge at low
pressure, there was a report about the effect of
oxygen concentration on atomic nitrogen
concentration of Kudrle et al. [5]. The authors
observed that until a certain threshold, the
concentration of atomic nitrogen rises rapidly with
the amount of O2 admixture. If more O2 admixture is
added, the concentration of nitrogen atoms decreases
sharply. There was also in 2000, for the discharge at
atmospheric pressure, Miralai et al. [6] mentioned
about the oxygen concentration of 500ppm as a
threshold to create atmospheric pressure glow
discharge (APGD) in the nitrogen discharge. The
total light emission from nitrogen APGD is mainly
composed of the second positive system of N2. Then,
adding the more oxygen gas exceeding the threshold
could suppress this light emission.
In this study, the influence of oxygen concentration
of the discharge at low pressure and at atmospheric
pressure on this nitrogen isotope separation process
is addressed.
2. Experimental
The experiments in this study were carried out
according to the setup in the schematic shown in
Figure 1. DC power supply was used to generate the
glow discharge. The reaction zone was a U-shaped
tube (7 mm I.D) made by Pyrex glass. Two stainless
steel electrodes Ф 1 mm in the reaction zone had a
15 cm gap between them. The low energy electrons
in the positive column are preferable for the
vibrational excitation. Therefore, we utilize
relatively long discharge tube. However, for the case
discharge at atmospheric pressure, we reduced the
discharge gap to 5 mm in order to be able to
generate the discharge.
The wall of the reaction U-tube was covered by the
liquid nitrogen coolant. Flow conditions were set for
all experiments. The discharge was generated
between the gap of two electrodes to form the nitric
oxide.
DC Power
Supply
P1
P2
Reduction zone
MS
N2 O2
Copper
Reaction zone
Vacuum
Pump
Liquid N2
Liquid N2
Cold Trap
Figure 1. Schematic of the experimental setup
The natural nitrogen gas and oxygen gas were
entered the reaction U-tube with a defined flow rate.
The products containing nitric oxides were produced
under the discharge. These products were supposed
to condense on the cold wall of the U-tube reactor
while the unreacted gases were pumped out with the
flow. When the time for generating the discharge
was finished, the reactor tube was closed and heated
up to room temperature. At this time, the pressure
(P1) inside reactor tube was recorded. The total
amount of products was determined by this pressure
(P1) of the products and the reactor volume. Since
the products contained many kinds of species, which
may affect the accuracy of the following analysis
process, they were converted to nitrogen molecules
again by the reduction reaction with copper threads
[7]. The reduction zone was located next to the
reaction zone and another U-shaped (5 mm I.D)
quartz tube was used. The raw copper threads were
placed inside this tube. The discharge products were
transferred from the reaction U-tube to the reduction
U-tube after measuring its pressure in reaction Utube at atmospheric temperature. Then, the reduction
zone was closed and this reduction reaction was
taken place at 800oC for five minutes. During the
reduction time, the products containing nitric oxides
reacted with the raw copper to release nitrogen gas
forming copper oxide. After that, the pressure (P2)
of the nitrogen gas in this reduction zone was
recorded. The nitrogen separation factor was
calculated based on the mass spectrometer
3. Results and Discussion
In order to evaluate the efficiency of the enrichment
of nitrogen isotope 15, the separation factor is
defined by this formula:
(15 N/ 14 N) products
(15 N/ 14 N) reagents
(3)
Another parameter that is used to evaluate the
amount of the collected nitrogen product in the
process is the conversion of nitrogen. The formula
for deriving this parameter is presented here:
θN2 =
2
Mole of collected nitrogen gas
.100% (4)
Mole of nitrogen gas fed
in which, the mole of collected nitrogen gas was
calculated based on the pressure (P2) and the volume
of the reduction zone at room temperature.
1.5
1
0.5
0
0
0.1
0.2
0.3
0.4
0.5
Oxygen Concentration (%)
Figure 3. Effect of oxygen concentration on separation factor
(atmospheric pressure)
0.2
Conversion of Nitrogen [x10 -4] (%)
β=
paper has reported. We hope we can make it in
another discussion.
Separation Factor
measurements of the nitrogen gas in the reduction
zone and the natural nitrogen gas. For further
purification, after the reduction process, the mixed
gases were passed by the cold trap 77 K before
entering the mass spectrometer (MS).
0.15
0.1
0.05
0
0
0.1
0.2
0.3
0.4
0.5
Oxygen Concentration (%)
Figure 2. Images of discharge at different oxygen concentration
(atmospheric pressure)
The images which display the discharge status at
different oxygen flow rate were placed in Figure 2.
The shape of the discharge at atmospheric pressure
was affected significantly by the oxygen flow rate.
At 0.5 sccm of oxygen flow rate, the discharge
probably turns to arc discharge.
The higher nitrogen separation factor was observed
at the smaller oxygen concentration as in Figure 3.
There is an agreement from this result with the
previous study. The higher oxygen concentration
reduces the efficiency of nitrogen isotope separation
process. However, we still do not determine the
threshold value of oxygen concentration as some
Figure 4. Effect of oxygen concentration on conversion of
nitrogen (atmospheric pressure)
The conversion of nitrogen was calculated and
presented in Figure 4. This results show the rather
small value of conversion of nitrogen. The
conversion of nitrogen increases reasonably with the
increase in oxygen concentration. However, from
Figure 3., the isotope selectivity of the chemical
reaction reduces as the oxygen concentration rises.
Figure 5. shows the images of discharge tube during
and after the discharge generation at low pressure.
Since there is almost no change of the shapes of
discharge light with different oxygen concentration,
thus, this figure presents the general discharge at low
pressure. The blue color in the second image is the
ozone condensation. Since the glass tube of N2-O2
Conversion of Nitrogen (%)
discharge was cooled by liquid nitrogen, the ozone
liquid was formed from the oxygen supply during
the discharge.
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
During
discharge
After
discharge
Figure 5. Images of discharge tube (low pressure)
Separation Factor
The effect of oxygen concentration on the nitrogen
separation factor in case of discharge at low pressure
is observed in Figure 6. The separation factor has a
maximum at value of 5%. This tendency is different
from that of atmospheric discharge system. The
discrepancy would be attributed to the rapid oxygen
conversion into ozone liquid. As shown in Figure 5.,
oxygen is converted to ozone rapidly in the
discharge tube at low pressure systems with liquid
nitrogen coolant. Therefore, feed oxygen
concentration can not reflect the practical oxygen
concentration in the discharge tube.
10
20
Oxygen Concentration (%)
Figure 7. Effect of oxygen concentration on conversion of
nitrogen (low pressure)
4. Conclusion
1.6
The effect of oxygen concentration in the discharge
at the low pressure and atmospheric pressure on the
separation factor and the yield of nitrogen gas
product have been investigated in this study. The
higher oxygen concentration gives the lower
nitrogen separation factor in the discharge at both
low pressure and atmospheric pressure. The highest
separation factor of 1.8 was observed at 0.03% of
oxygen concentration in the discharge at
atmospheric pressure. The conversion of the
nitrogen was low because a very low oxygen
concentration was used in the process.
1.5
References
1.4
1.3
1.2
1.1
1
0
5
10
15
20
Oxygen Concentration (%)
Figure 6. Effect of oxygen concentration on separation factor
(low pressure)
The conversion of nitrogen at low pressure is shown
in Figure 7. There is the same tendency of
conversion of nitrogen of discharge at low pressure
and at atmospheric pressure although the conversion
in the atmospheric pressure system is much smaller
than that of low pressure system.
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