BHATIA SURESH K (AU)
KUMAR A V ANIL (AU)
BHATIA SURESH K (AU)
| US6332914B1 | 2001-12-25 |
PATENT ABSTRACTS OF JAPAN
CLAIMS
1. A method of separating isotopes, including: providing a fluid containing a mixture of the isotopes, preparing a filter system having a pore size at room temperature which is generally greater than the molecular diameters of the isotopes, cooling the mixture and the filter system to a temperature at which quantum effects differentially enlarge the diameters of the isotopes and differentially effect their passage through the filter, and passing the mixture once or more through the filter system to separate the isotopes.
2. A method according to claim 1 wherein the isotopes are H 2 and D 2 , and the filter system includes zeolite rho.
3. A method according to claim 2 wherein the temperature is in the range 1OK to IOOK and preferably about 20 to 80K and most preferably about 30K.
4. A method according to claim 2 wherein the filter includes one or more membranes of zeolite rho.
5. A system for separation of the isotopes H 2 and D 2 , including: a reservoir containing a mixture of the isotopes to be separated, a pair of collectors for respective isotopes when separated, and a temperature controlled separator connected between the reservoir and the collectors, wherein the separator includes a filter arrangement having a pore size which differentially affects passage of the isotopes at the control temperature due to quantum swelling.
6. A system according to claim 5 wherein the filter arrangement includes one or more membranes of zeolite rho, aluminophosphate ALPO-25, aluminophosphate ALPO- 21, aluminosilicate Bikitaite, borosilicate RUB-24, or porous carbon materials.
7. A system according to claim 6 wherein the filter arrangement includes one or more membranes of zeolite rho. |
ISOTOPE SEPARATION BY QUANTUM SWELLING
FIELD OF THE INVENTION
This invention relates to separation of isotopes by quantum swelling effects and in particular but not only to separation of the hydrogen isotopes H 2 and D 2 .
BACKGROUND TO THE INVENTION
A range of techniques are known for separation of hydrogen isotopes, including thermal diffusion, centrifugation, laser separation, adsorption and proton exchange membranes. In general these methods are either expensive or inefficient. Microporous materials are suitable for separating many mixtures through size and shape selectivity but are generally unsuitable for hydrogen mixtures, because the hydrogen molecules (hydrogen H 2 , deuterium D 2 , tritium T 2 ) ordinarily have the same size and shape, differing only in mass. The diffusivities of H 2 and D 2 differ only by a factor of V2 at room temperature for example.
Diffusion of hydrogen molecules in microporous materials has been investigated theoretically in a classical framework using energies determined by the traditional Lennard
Jones potential. However, in narrow pores at low temperatures when the space available for movement of molecules becomes comparable to the de Brogue wavelength of the molecules, quantum effects can become significant. The correction to the classical potential depends inversely on temperature and molecular mass, and differences between the effective size of different hydrogen isotopes or other isotopes can arise.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an alternative method for separation of hydrogen isotopes at low temperatures, and also possibly other isotopes with similarly small molecular mass.
In one aspect the invention may be said to reside in a method of separating isotopes, including: providing a fluid containing a mixture of the isotopes, preparing a filter system having a pore size at room temperature which is greater than the molecular diameters of the isotopes, cooling the mixture and the filter system to a temperature at which quantum effects differentially enlarge the diameters of the isotopes and differentially effect their passage through the filter, and passing the mixture once or more through the filter system to separate the isotopes.
In one embodiment the isotopes are H 2 and D 2 , and the filter system includes zeolite rho. Preferably the temperature is in the range 1OK to 10OK, more preferably 20 to 8OK, and most preferably about 30K. The filter generally includes one or more membranes of zeolite rho. A range of other isotopes with small molecular mass such T 2 , 3 He, 4 He may be separated from various mixtures by a method of this general kind, using a range of different filter systems.
In another aspect the invention resides in a system for implementing a method as defined above. The invention may further be said to reside in any alternative combination of features that are indicated in this specification. All equivalents of these features are deemed to be included.
LIST OF FIGURES
Preferred embodiments of the invention will be described with reference to the accompanying drawings, of which: Figure 1 shows the Lennard- Jones potential for H 2 and D 2 at 50K,
Figure 2 shows temperature variation of the LJ size parameter σ for H 2 and D 2 , Figure 3 schematically indicates quantum swelling of H 2 and D 2 at low temperatures,
Figure 4 shows the structure and pore size of zeolite rho, Figure 5 shows selectivity data for D 2 over H 2;
Figure 6 summarises data for various isotopes and temperatures, and
Figure 7 outlines a separation system.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawings it will be appreciated that the invention may be implemented in various forms, and that this description is given by way of example only. The invention is described primarily in relation to separation of hydrogen isotopes using zeolite rho as a porous material, although it will be appreciated that other isotopes and materials may also be suitable. Full details of the separation system have not been given but will be known to a skilled person.
Figure 1 shows Lennard Jones (LJ) potential for interactions between gaseous H 2 or D 2 and an oxygen atom in the solid phase of a microporous material. Oxygen provides the bulk of the interaction potential with a gaseous host in a zeolite. In this figure U is the potential energy, kβ is the Boltzmann constant, r the distance between a gas molecule and solid atom, and σ is the distance between the gas molecule and the solid atom at the position where the potential changes from being attractive to repulsive. Both the gas molecule and solid atom are assumed spherical. The solid line is the classical potential which is identical for both H 2 and D 2 . The dashed and dotted lines respectively include their quantum corrections at 50K. The different values of r at U=O for example indicate that the effective size of H 2 is larger than D 2 at this temperature, an effect which is known as "quantum swelling" [1-3].
Figure 2 shows temperature variation in the effective LJ size parameter σ for H 2 and D 2 after the quantum correction is made. Computer simulations have determined that the critical pore size that the gas molecule can enter is roughly 0.96σ. The value of σ increases as temperature decreases with the increase being greater for H 2 than D 2 . This leads to an increasing steric hindrance which reduces the diffusivity of H 2 over D 2 through narrow pores which are about the same size as the molecules. A microporous material having pores of a suitable size can therefore be effective as a filter so long as the temperature is controlled within an appropriate range.
Figure 3 shows the effect schematically in relation to an ideal pore of diameter d, where the effective value of σ at room temperature (about 20 0 C) for both H 2 than D 2 is 0.2782 nm, but swells to 0.291 nm and 0.285 nm respectively at 100 K. If d is 0.28 nm, which corresponds approximately to zeolite rho, then D 2 will pass through the pore relatively easily while H 2 will be relatively hindered. In quantum terms, the effect may be considered to arise when the uncertainty in position of the molecule, as estimated by the de Brogue wavelength λ for example, is comparable to (d - 0.96σ) where σ is the room temperature value. A reasonable condition the effect to be economic for purposes of isotope separation is thought to be given by λ/(d - 0.96σ) >10.
Figure 4 shows the zeolite rho cage which presents a window having a pore size of 0.543 nm measured centre to centre between oxygen atoms as indicated. After subtracting the diameter of the oxygen atom as 0.2644 nm the pore size is 0.2786 nm. At 100 K the value of σ for H 2 has increased to 0.29 nm so that 0.96σ is 0.2784 nm which approximately matches the pore size. At still lower temperatures passage of the H 2 molecule through the cage is increasingly hindered. Zeolite rho can therefore be used as a filter material in separating H 2 and D 2 although a range of other materials will also be suitable such as aluminophosphates ALPO-25, ALPO-21, aluminosilicate Bikitaite, and borosilicate RUB- 24, for example. Porous carbon materials have been shown to be useful in some circumstances [4-5].
Figure 5 shows the variation of kinetic selectivity with temperature for H 2 and D 2 in zeolite rho determined by computer simulation. Kinetic selectivity is defined as the ratio of the transport diffusivity of D 2 to H 2 with the closed and open circles in the main figure demonstrating the difference between classical and quantum corrected calculations respectively. Their values are similarly low towards room temperature where quantum effects are negligible, and are also thought to be low at very low temperatures where both H 2 and D 2 are effectively swollen beyond the pore size and are similarly hindered. At around 10-100 K however, preferably 20-80K, and especially at around 3OK, the pore size lies between the effective size of the two isotopes, so that D 2 diffuses relatively easily while H 2 does not. The inset to this figure shows the flux selectivity, defined as the product
of the equilibrium constant and the kinetic selectivity, and provides the flux ratio when the gas phase concentration gradient is the same for each isotope. Open triangles and closed triangles represent flux selectivity based on quantum and classical considerations respectively.
Figure 6 gives specific σ and λ values for various isotopes at room temperature, 10OK, 65K, 4OK and 30K, by way of example. It can be seen that a separation system having a suitably selected filter material could be effective in separating a range of different mixtures. A recent experiment has shown that a mixture of H 2 and D 2 can be effectively filtered on porous carbon medium at around 77K [5].
Figure 7 shows a system that could be used for separation of hydrogen isotopes. It includes a filter in which a membrane module having one or more zeolite rho membranes supported on a porous ceramic such as alumina. The support would be in the form of tubes, typically about 10 mm in outer diameter and coated on the inner surface with a zeolite rho layer typically about 1-30 microns thick. The tube length may be in the range of 0.5 to 2 meters, for a feedstock reservoir having a volume of 100-400 lit and pressure as high as 300 bar. The membrane separator will be kept at a low temperature, in range of 20 to 80 K. Feedstock from the reservoir flows through the tubes, and the permeate collected from the shell side of the membrane in the permeate reservoir. The permeate will be richer in deuterium as only a small fraction of the hydrogen will flow through into the permeate, while essentially all the deuterium will flow through. The vacuum pump will maintain low pressure on the permeate side, and a pressure controller will control the pressure in the feed side.
Typically, in nature most mixtures have about 1 part of deuterium per 5000 parts of hydrogen, and the process will enhance the deuterium to hydrogen ratio by a factor of 20- 30 in a single pass. The product gas pressure will be boosted after the vacuum pump to a suitable level for storage in the collection vessel. When sufficient product has been collected it can be further concentrated by a factor of 20-30 by again passing through the membrane module. This process can be repeated till sufficient purity of deuterium has
been obtained. For example, even based on a conservative estimate of concentration by a factor of 20, after four steps deuterium of 97 percent purity may be obtained, while after five steps deuterium of over 99.8 percent purity may be obtained.
Large flow rates of hydrogen are available in ammonia plants where hydrogen is an important feedstock for ammonia synthesis. Use of such a membrane device in an ammonia plant will provide an economical application of the method to concentrate and produce deuterium. In such situations the feedstock reservoir is not necessary, and the process runs on a continuous flow of hydrogen from its source.
REFERENCES (not to be considered part of the common general knowledge).
1. X-P Jiang and M. Cole, "Quantum contribution to Henry's law of adsorption", Physical Review B, vol 33, p2803 (1986).
2. N. Tchouar, F. Ould-Kaddour and D. Levesque, "Computation of the properties of liquid neon, methane, and gas helium at low temperature by the Feynman-Hibbs approach", Journal of Chemical Physics, vol 121, p7326 (2004).
3. H. Tanakaet al., "Quantum effects on hydrogen adsorption in internal nanospaces of single wall carbon nanohorns", Journal of Physical Chemistry B, vol 108, 17457 (2004).
4. C. Bai, M-D Jia, J.L. Falconer and R.D. Noble, "Preparation and Separation Properties of Silicalite Composite Membranes", Journal of Membrane Science, vol. 105 (1995), p79-87.
5. X Zhao, et al, "Kinetic Isotope Effect for H 2 and D 2 Quantum Molecular Sieving in Adsorption/Desorption on Porous Carbon Materials", J Phys Chem B 2006, 110, 9947- 9955.
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