Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
2002-05-07
2003-11-25
Wong, Edna (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
Reexamination Certificate
active
06653587
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the separation of a desired isotope from polyatomic molecules containing different isotopes, by applying to the molecules in the gas phase at a predetermined pressure, near infrared radiation of a first pulsed laser and, after a predetermined time-lag which allows a sufficient number of collisions, infrared radiation of a second pulsed laser of different frequency to produce a chemical reaction resulting in a molecule, enriched in the desired isotope, which can be separated from the remainder of the material. The invention is exemplified in a particular by the separation of
13
C isotopes in polaytomic molecules consisting of mostly
12
C isotopes and which contain C—H and C—F bonds.
BACKGROUND OF THE INVENTION
The stable
13
C isotope has been widely used in many applications but until recently in relatively small volume. Recent medical development of the so-called carbon-
13
Diagnostic Breath Test (
13
C DBT) (U.S. Pat. No. 4,830,010) has dramatically changed the situation. The DBTs are used to assess the condition of organs of the human digestive system. Because of its safety, relative simplicity and wide range of application, the DBT technology has rapidly increased the demand for
13
C.
A limiting factor for the growth in the use of DBTs is the relatively high production cost of highly (>99%) isotopically pure
13
C. The bulk of the
13
C at present is produced by multi-cycle low temperature distillation of CO. This technique is well developed and has nearly reached the maximum of its efficiency, limited by its high energy consumption.
The molecular laser isotope separation (MLIS) approach provides an alternative for production of high purity stable isotopes. The most developed method for MLIS of
13
C is based on infrared multiphoton dissociation (IRMPD) of CF
2
HCl by a pulsed CO
2
laser. This method relies on a 20 cm
−1
isotopic shift in the IR absorption spectrum of the
13
C containing molecules relative to
12
C containing molecules for selective absorption and dissociation of
13
CF
2
HCl. The CF
2
dissociation fragments recombine, resulting in stable C
2
F
4
molecules that are separated from parent molecules by distillation.
An example of a recent implementation of this approach by Ivanenko et al. (Applied Physics B, 62, pp. 329-332, 1996) produce a macroscopic enrichment of
13
C using a high-power high repetition rate industrial CO
2
laser. A report by V. Y. Baranov et al., (Proceedings of 4th All-Russian International Scientific Conference on “Physical Chemical Processes at Selection of Atoms and Molecules”, 1999, pp. 12-16) describes a near completed pilot plant in Kaliningrad, Russia, which is designed to produce several tens of kilograms of isotopically pure
13
C a year using the same approach. In both cases, CF
2
HCl is enriched to 30-50% in
13
C by selective IRMPD. In both cases, it is suggested that further enrichment of the products up to 99%
13
C could be accomplished by non-laser techniques such as centrifugation. In another approach, a second stage of laser separation is employed to bring partially enriched product to higher levels of enrichment (Ph. Ma et al., Appl. Phys. B 49 503 (1989)). In the case of
13
C isotope separation using CF
2
HCl as a starting material, the partially enriched product (C
2
F
4
) is chemically converted to a molecule suitable for the next laser isotope separation cycle (A. P. Dyad'kin, et al.; Proceedings of 4th All-Russian International Scientific Conference on “Physical Chemical Processes at Selection of Atoms and Molecules”, 1999, pp. 17-20). This extra stage complicates the overall process and significantly increases cost of the product.
Under certain conditions, single-laser IRMPD of CF
2
HCl has demonstrated the capability of producing products highly enriched in
13
C in a single stage, but this high degree of enrichment comes at the cost of productivity. The work of Gauthier et al. (Appl. Phys. B. 28, 2, 1982) achieves enrichment to 96%, but this requires operating at low laser fluence and low pressure, both of which decrease he productivity. Reasonable productivity is achieved at only 50%
13
C enrichment, which falls short of the high purity (>99%) required for medical applications.
One approach to increase the selectivity in laser isotope separation is to use a single-stage two-laser process. U.S. Pat. No. 4,461,686 relates to two-color IR—IR MLIS wherein a first laser excites a non-specified vibrational state and a second laser excites molecules up to a level of a chemical conversion, including dissociation. A similar method has been successfully realized on a laboratory scale by Evseev et al. (Appl. Phys. B36, 93, 1985; Sov. J. Quantum Electron. 18, 385, 1988). While this approach overcomes some of the drawbacks of a single-laser process and achieves relatively high selectivity (S=6000 which corresponds to
13
C enriched to 98.5%), low pressure is still required, limiting the productivity.
One widely known problem of two-laser isotope separation schemes is the possibility of vibrational relaxation of the molecules in the time between the two laser pulses, leading to loss of isotopic selectivity, U.S. Pat. No. 4,461,686 clearly states this problem by specifying a time delay between laser pulses that is shorter than the vibrational relaxation time but longer than the rotational relaxation time of the polyatomic molecules, allowing time for rotational but not for vibrational relaxation.
A number of other two-laser schemes have been employed for separation of various isotopic species, but in most cases, conditions are adjusted to minimize collisions in the time between the two laser pulses and/or the deleterious effects of collisions on the selectivity is explicitly mentioned. In their two-color infrared isotopically selective decomposition of UF
6
, Rabinowitz et al. (Optics Letters 7, 212 (1982)) indicate that they use pressures of less than 10
−7
Torr during runs, ensuring collision free reactions. They clearly state that energy-exchange collisions between the two isotopic species may scramble the selectivity. Using a similar two-color laser isotope separation scheme for SeF
6
, Tiee and Wittig (J. Chem. Phys. 69, 4756 (1978)), state that they use a delay between the two lasers that is short enough so that deleterious energy transfer processes do not have a chance to interfere. In their two-color multiple photon dissociation of CF
3
T, Pateopol and O-Neil (Laser Isotope Separation, SPIE, Vol. 1859, p. 210-218 (1993)) show in
FIG. 4
that an increase in pressure, which increases the frequency of collisions, decreases the isotopic selectivity. In a two laser scheme for separation of sulfur isotopes, French patent FR2530966A does not explicitly mention collisions but uses sufficiently low pressure and short time delay such that vibrational relaxation from collisions between the two laser pulses is minimized. In their two laser dissociation scheme for OsO
4
, Ambartzumian et al. (Optics Letters 1, 22 (1977)) do not mention collisions, however the information they provide on the experimental conditions, particularly the low pressure (~0.3 Torr) suggests that no collisional vibrational relaxation occurs during the process.
A few studies have observed that under certain conditions, collisions seem to enhance the isotopic selectivity. In their single-laser IRMPD of CF
2
HCl for
13
C enrichment, Gauthier et al. (Appl. Phys. B. 28, 2, 1982,
FIG. 3
) demonstrate increasing selectivity with increasing pressure. This increase in selectivity is accompanied with a corresponding decrease in dissociation efficiency (also FIG.
3
), leading to low values of productivity. In their two-laser IRMPD studies of CF
2
HCl for
13
C enrichment, Evseev et al. (Appl. Phys. B36, 93, 1985; Sov. J. Quantum Electron. 18, 385, 1988) observe modest increase in selectivity both upon increase in the pressure of the working gas as well as upon increasing the delay between the two lasers. They attribute the increased selectivity to different rates of vibra
Boiarkine Oleg
Rizzo Thomas
Ecole Polytechnique de Lausanne (EPFL)
Sturm & Fix LLP
Wong Edna
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