Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-01-27
2001-11-27
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06321565
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing water (heavy oxygen water) highly enriched in
17
O or
18
O (hereinafter referred to as the heavy isotopes of oxygen) which are isotopes of oxygen present in water. More specifically, the present invention relates to a method in which oxygen is enriched in
16
O
17
O,
16
O
18
O,
17
O
17
O,
17
O
18
O, and
18
O
18
O by means of cryogenic distillation, then water is produced from the enriched product, and then water which is further enriched in the oxygen isotope
17
O or
18
O is produced by means of water distillation.
In addition, the present invention relates to a method of producing water which is enriched in the oxygen isotope
17
O or
18
O, and which has a constitution in which the hydrogen isotopes are present in a ratio approximating their natural abundance ratio. In addition, alternatively, the present invention relates to a method of producing water having a hydrogen isotope constitution which contains deuterium (D) in a ratio lower than the natural abundance ratio. In addition, alternatively, the present invention relates to a method of producing water having a hydrogen isotope constitution which contains deuterium (D) in a ratio higher than the natural abundance ratio.
This application is based on patent application No. Hei 11-23148 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Natural oxygen contains
16
O at a ratio of 99.7591% (atomic percent, hereinafter the same),
17
O at a ratio of 0.037%, and
18
O at a ratio of 0.204%.
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 research of oxygen compounds using nuclear magnetic resonance and the like.
As enrichment methods for these heavy isotopes of oxygen, 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, CO or oxygen as the starting material.
However, the above mentioned conventional techniques have the following problems.
In the above-mentioned water distillation method, it is easy for water (HD
16
O) containing the light isotope (
16
O) and deuterium to be mixed with the heavy isotope-enriched product.
FIG. 14
shows an example of enrichment in heavy isotopes of oxygen by means of water distillation using water containing oxygen in which each of the isotopes is present in the above-mentioned natural abundance ratios. In this example, the fraction obtained at the bottom of the column which was enriched in H
2
18
O to approximately 1.1% was also enriched in HD
16
O to approximately 0.2%.
Since the relative volatility of H
2
16
O/HD
16
O is greater than the relative volatility of H
2
16
O/H
2
18
O, enrichment in HD
16
O is comparatively easier to carry out. Therefore, it is believed that in the above-mentioned example, enrichment in HD
16
O hinders enrichment in H
2
17
O and H
2
18
O.
Dostrovsky et al reported obtaining H
2
18
O of 99% or greater by means of water distillation (I. Dostrovsky and M. Epstein, “The production of stable isotopes of oxygen” Analytical Chemistry Symposia Series, Vol. 11, pp. 693-702 (1982)). However, as mentioned above, when enriching in H
2
17
O and H
2
18
O, usually, enrichment in water molecules (HD
16
O, etc.) containing
16
O and deuterium also occurs at the same time, this hinders the enrichment in H
2
17
O and H
2
18
O, and it is difficult to industrially obtain H
2
18
O having a purity of 99% or greater. The purity of commercially available H
2
18
O is approximately 97%.
In addition, because the latent beat of vaporization of water is comparatively large (for example, it is about six times greater than that of oxygen), the apparatus for water distillation is comparatively large and energy consumption is great. For this reason, there is a tendency for the apparatus costs and operation costs to be large for water distillation methods.
In NO distillation methods as well, there is the problem that it is relatively easy for the compound NO (
15
N
16
O) which contains the heavy isotope of nitrogen and the
16
O to be mixed with the obtained heavy isotope-enriched product, and it is difficult to obtain an enriched product having a high concentration of heavy oxygen isotopes.
In oxygen distillation, since the abundance ratio of
17
O and
18
O in natural oxygen is low, the abundance ratio of
17
O
17
O,
17
O
18
O and
18
O
18
O is extremely low. For this reason, the major proportion of oxygen molecules which contain heavy isotopes are
16
O
17
O and
16
O
18
O.
In this way, since most heavy isotope containing-oxygen molecules contain
16
O, in oxygen distillation, even when the enrichment proceeds to close to 100% of heavy isotope containing-oxygen molecules, the enrichment rate for the heavy isotopes, that is
17
O and
18
O, is a low value of 50% or less.
SUMMARY OF THE INVENTION
In view of the above-mentioned situations, an object of the present invention is the provision of a method and an apparatus with which it is possible to carry out enrichment in the heavy isotopes of oxygen to a high concentration and which can be implemented at low cost.
The method of producing heavy oxygen water of the present invention comprises enriching an oxygen starting material, which contains heavy isotopes of oxygen, in heavy isotopes of oxygen by means of cryogenic distillation; and producing water containing a high concentration of the above-mentioned heavy isotopes of oxygen by adding hydrogen to the enriched product and reacting them.
In addition, the method of producing heavy oxygen water of the present invention comprises enriching an oxygen starting material, which already contains heavy isotopes of oxygen, in heavy isotopes of oxygen by means of cryogenic distillation, then forming water containing a high concentration of the above-mentioned heavy isotopes of oxygen by adding hydrogen to the enriched product and reacting them, and producing heavy oxygen water enriched in the above-mentioned heavy isotopes of oxygen by means of the distillation of the formed water.
In addition, in the method of the present invention, the above-mentioned heavy isotopes of oxygen may be either of
17
O or
18
O or they may be both
17
O and
18
O.
In addition, the method of the present invention comprises conducting the above-mentioned cryogenic distillation of oxygen and the above-mentioned distillation of the formed water using a distillation column packed with structured packing.
In addition, the method of the present invention comprises enriching oxygen in the molecular isotopes of oxygen of
16
O
18
O,
16
O
17
O,
17
O
17
O,
17
O
18
O, and
18
O
18
O, wherein, as the method for the above-mentioned cryogenic distillation of oxygen, 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 gas (vapor) in a direction at right angles to the above-mentioned main flow direction is promoted and mass transfer is accomplished.
In addition, in the method of the present invention, the cryogenic distillation of oxygen is conducted such that the density corrected superficial velocity (the superficial F factor) within the column is 0.5 m/s(kg/m
3
)
1/2
or greater and 2.0 m/s(kg/m
3
)
1/2
or less, and preferably 0.8 m/s(kg/m
3
)
Hayashida Shigeru
Kawakami Hiroshi
Kihara Hitoshi
Tachibana Hiroshi
Burns Doane , Swecker, Mathis LLP
Doerrler William
Drake Malik N.
Nippon Sanso Corporation
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