Chemistry of inorganic compounds – Oxygen or compound thereof
Reexamination Certificate
2000-07-10
2002-10-08
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
C423SDIG007, C062S919000, C203S005000, C203S071000, C202S158000, C202S172000, C422S186220
Reexamination Certificate
active
06461583
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for enriching oxygen in the heavy oxygen isotopes,
17
O and
18
O; and in particular, the present invention relates to a method and apparatus for enriching oxygen in these heavy oxygen isotopes by means of cryogenic distillation.
In addition, the present invention relates to a method and apparatus for further concentrating heavy oxygen isotopes by means of conducting isotope scrambling following the cryogenic distillation.
This application is based on patent application No. Hei 11-150733 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
Naturally abundant oxygen comprises 99.759% (atomic %, used in this way hereinafter) of
16
O, 0.037% of
17
O, and 0.204% of
18
O.
Among these, the heavy isotope
18
O is used as a tracer in fields such as agriculture, biology, and medicine.
In addition, in the same way, since the heavy isotope
17
O has nuclear magnetic moment, it is used in the research of oxygen compounds using nuclear magnetic resonance and the like.
As enrichment methods for these heavy oxygen isotopes, there are distillation, thermal diffusion, chemical exchange (reactions), and the like. However, as a method of production with low cost and high volume, distillation is generally used. As the distillation method, there are methods which use water, NO, or CO as the starting material.
Among these method, as those methods whose success has been proven, water distillation methods using water as the starting material, and NO distillation methods using NO as the starting material can be mentioned.
As a water distillation method; the method practiced by Dostrovsky et al is known, and they reported that using this method it was possible to produce approximately 6 kg of
18
O of a concentration of 98 to 99% in a year, and 1.5 kg of
17
O of a concentration of 25% in a year. In addition, there are attempts to obtain high concentrations of
17
O by means of further enrichment of
17
O obtained by means of this method using a thermal diffusion method.
Since NO has a higher relative volatility compared with other starting materials, the enrichment efficiency for the above-mentioned isotopes in NO distillation methods is highly advantageous.
This method is used widely for enrichment of the isotopes of nitrogen and, normally, the above-mentioned heavy oxygen isotopes are obtained as bi-products of enrichment of the isotopes of nitrogen.
However, the above-mentioned conventional techniques have the following problems.
As shown in Table 1 and Table 2, since heavy isotopes are present in hydrogen and in nitrogen, there is the problem that in the above-mentioned water distillation methods, enrichment of water comprising the light isotope of oxygen (
16
O) and the heavy isotope of hydrogen occurs, and in NO distillation methods, enrichment of NO comprising the light isotope of oxygen (
16
O) and the heavy isotope of nitrogen occurs.
More specifically, in water distillation methods, it is easy for water containing the light isotope of oxygen (
16
O) and deuterium (HD
16
O, etc.) to become mixed into the obtained heavy isotope enriched product. This hinders enrichment of the H
2
17
O and H
2
18
O which contain the heavy isotopes of oxygen, and it is difficult to industrially obtain product which is highly enriched in the heavy isotopes of oxygen, such as H
2
18
O having a purity of 99% or greater. The purity of commercially available H
2
18
O is approximately 97%.
TABLE 1
Mass number
Water molecule
Abundance ratio
18
H
2
16
O
0.99728
19
H
2
17
O
0.00037
19
HD
16
O
0.00031
20
H
2
18
O
0.00204
20
HD
17
O
1.15 × 10
−7
20
D
2
16
O
2.43 × 10
−8
21
HD
18
O
6.36 × 10
−7
21
D
2
17
O
9.00 × 10
−12
22
D
2
18
O
4.96 × 10
−11
TABLE 2
Mass number
NO molecule
Abundance ratio
30
14
N
60
O
0.99390
31
14
N
17
O
0.00037
31
15
N
16
O
0.00369
32
14
N
18
O
0.00203
32
15
N
17
O
1.37 × 10
−6
33
15
N
18
O
7.55 × 10
−8
In addition, since the latent heat of vaporization of water is comparatively high (e.g., approximately six times that of the latent heat of vaporization of oxygen), the water distillation apparatus is comparatively large and energy consumption is great. For this reason, there is a tendency for water distillation methods to have increased apparatus and operational costs.
In addition, in NO distillation methods, it is easy for NO (
15
N
16
O) containing the heavy isotope of nitrogen and oxygen (
16
O) to become mixed into the obtained heavy isotope enriched product, and there is the problem that it is difficult to obtain an enriched product which is highly enriched in heavy oxygen isotopes.
In addition, due to reasons such as NO being a corrosive and poisonous gas, there is the problem that the above-mentioned NO distillation methods require a great deal of expense to put into practice.
SUMMARY OF THE INVENTION
In order to solve the above-mentioned problems, the method of the present invention provides a method of enrichment of heavy oxygen isotopes comprising enriching an oxygen starting material which contains heavy oxygen isotopes in at least one type of oxygen molecule selected from
16
O
17
O,
16
O
18
O,
17
O
17
O,
17
O
18
O and
18
O
18
O, which contain heavy oxygen isotopes, by means of cryogenic distillation of the oxygen starting material which contains heavy oxygen isotopes.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes comprising enriching an oxygen starting material which contains heavy oxygen isotopes in at least one type of oxygen molecule selected from
16
O
17
O,
16
O
18
O,
17
O
17
O,
17
O
18
O and
18
O
18
O, which contain heavy oxygen isotopes, by means of cryogenic distillation in which the oxygen starting material which contains heavy oxygen isotopes is supplied to a distillation column packed with structured packing.
In addition, in the method of enrichment of heavy oxygen isotopes of the present invention, as the method for the above-mentioned cryogenic distillation, a distillation method is used which comprises supplying an oxygen starting material to a distillation column which has been packed with structured packing; bringing about vapor-liquid contact between a descending liquid and an ascending vapor mainly on the surface of the above-mentioned structured packing within the above-mentioned distillation column; at which time, the liquid and the vapor flow in mutually opposite directions over the surface of the above-mentioned structured packing along the main flow direction, which is along the direction of the column axis, and at the same time mixing of the liquid and/or the vapor in a direction at right angles to the above-mentioned main flow direction is promoted and mass transfer occurs.
In addition, according to the present invention, it is preferable to perform the aforementioned cryogenic distillation of oxygen such that the density corrected superficial velocity (the superficial F factor) is at least 0.5 m/s(kg/m
3
)
½
and no greater than 2.0 m/s(kg/m
3
)
½
and more preferably, at least 0.8 m/s(kg/m
3
)
½
and no greater than 1.8 m/s(kg/m
3
)
½
.
In addition, according to the present invention, it is preferable to perform the aforementioned cryogenic distillation of oxygen such that the distillation pressure is in the range of 0.5 bar to 5 bar, and more preferably, 1.1 bar to 2.5 bar.
As the oxygen starting material, it is preferable to use highly pure oxygen having a purity of 99.999% or greater. In particular, it is preferable to use cryogenically manufactured high purity oxygen obtained from a high purity oxygen preparation device using cryogenic distillation.
In addition, the method of the present invention is a method for enrichment of heavy oxygen isotopes comprising using a distillation column comprising three distillation columns, a first column, a second column and a third column, as the above-mentioned distillation column; supply
Egoshi Nobuaki
Hayashida Shigeru
Kawakami Hiroshi
Kihara Hitoshi
Langel Wayne A.
Medina Manbel
Nippon Sanso Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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