METHODS AND SYSTEMS FOR SPATIALLY SEPARATING OR DISTRIBUTING ISOTOPES

Abstract

Methods and related systems for separating isotopes of an element are provided. The element has at least two isotopic forms. The method includes hyperpolarizing one or more of the isotopic forms in a feedstock, and applying a magnetic field to the target isotopes in order to at least partially spatially separate the isotopic forms of the element from one another.

Claims

1) A method of separating isotopes of an element, said element having at least two isotopic forms, the method comprising: hyperpolarizing one or more of said isotopic forms in a feedstock; and applying a magnetic field to the target isotopes in order to at least partially spatially separate the isotopic forms of the element from one another.

2) The method of claim 1, where hyperpolarization is produced by cooling the isotopic forms in the presence of a magnetic field.

3) The method of claim 2, where the isotopic forms are cooled to at or below about 10 K in temperature and the magnetic field is at or above about 1 Tesla.

4) The method of claim 3, where an adulterant is added to the frozen element to hasten T1 in a brute force environment.

5) The method of claim 3, where a quantum relaxation switch is used to hasten hyperpolarization of target isotope.

6) The method of claim 1, where at least one isotope of said element has a non-zero nuclear spin.

7) The method of claim 1, wherein said element is carbon.

8) The method of claim 7, wherein said element is carbon in the form of carbon dioxide.

9) The method of claim 7, wherein said element is carbon in the form of carbon monoxide.

10) The method of claim 7, wherein said element is carbon in the form of methane

11) The method of claim 1, wherein said spatial separation of isotopic forms takes place in a liquid state.

12) The method of claim 1, wherein said spatial separation of isotopic forms takes place in a gaseous state.

13) The method of claim 1, wherein said spatial separation of isotopic forms takes place in a boundary between the liquid and gaseous states.

14) The method of claim 1, wherein a magnetic field gradient is used to facilitate the separation step.

15) The method of claim 14, wherein the magnetic field gradient is pulsed in time.

16) The method of claim 1, wherein the percentage of .sup.13CO.sub.2 in the feedstock CO.sub.2 is <1%.

17) The method of claim 1, wherein the percentage of .sup.13CO.sub.2 in the feedstock CO.sub.2 is <10%.

18) The method of claim 1, wherein the percentage of .sup.13CO.sub.2 in the feedstock CO.sub.2 is <50%.

19) The method of claim 1, wherein the percentage of .sup.13CO.sub.2 in the feedstock CO.sub.2 is <100%.

20) The method of claim 1, wherein differing nuclear polarization levels in an ensemble of more than one target isotope are produced by waiting a specified period of time for the nuclear polarization of one isotope to decay away to a smaller value than that of the other isotope or isotopes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a chart illustrating a brute force (high B/T) polarization of .sup.13C.

[0021] FIG. 2 illustrates the spatial separation of .sup.13CO.sub.2 from .sup.12CO.sub.2 produced by polarized .sup.13CO.sub.2 molecules attracted towards field center of an ambient magnet (not shown).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Reference will now be made in detail to the present preferred embodiments of the disclosure. The method and corresponding steps of the disclosure will be described in conjunction with the detailed description of the system.

[0023] Materials containing non-zero nuclear magnetic moments have, in an external magnetic field, at least two energy states. These states are characterized in the art as up or down in reference to the direction of the external magnetic field vector, and the polarization P is defined as:

P=N.sub.upN.sub.down/N

where N=N.sub.up+N.sub.down. Nuclear polarization is a function of ambient temperature T and magnetic field B under the equation P=tan h(B/kT) where is the nuclear gyromagnetic ratio and k is Boltzmann's constant. Under ordinary equilibrium circumstances of T300 K and B<<1 T the nuclear polarization of ensemble of .sup.13C atoms is very small, less than a few parts per million.

[0024] Hyperpolarization refers to one or more techniques whereby the nuclear magnetic polarization P is temporarily enhanced, often by many orders of magnitude, above equilibrium. Techniques that are known in the art include laser polarization, dynamic nuclear polarization (DNP), Parahydrogen Induced Polarization (PHIP), and Brute Force (BF). Note that in the description that follows, any of the hyperpolarization techniques currently known in the art can be used to produce isotopic separation, with brute force being one particularly preferred embodiment.

[0025] Because the hyperpolarized state is by definition out of equilibrium, there is always a characteristic relaxation time wherein the system relaxes back to thermal equilibrium. In the art this time is known as T1. T1 can differ because of many factors including chemical composition, temperature, external magnetic field, etc. Generally polarization varies with time as Pexp (t/T1).

[0026] In addition to having different masses, isotopes often have different nuclear magnetic moments and/or different T1s. As a particular non exclusive example, the rare and valuable isotope of carbon .sup.13C has a nuclear spin of , while the most common isotope of carbon .sup.12C has a nuclear spin of 0.

[0027] Nuclei with non zero nuclear magnetic moments, such as .sup.13C, .sup.15N, .sup.129Xe, .sup.3He etc, are paramagnetic. Under ordinary circumstances, the magnetic moment of these nuclei is extremely small, much too small to be useful in producing isotopic enrichment. However, when polarized, the nuclear magnetic moment can be temporarily much larger.

[0028] For example, the dipolar field of .sup.129Xe, polarized to 18% and with a concentration of 1.5 M/liter has been measured to be 0.46 microTesla. This corresponds to a nuclear magnetization per unit volume of M/V78 T/m3. .sup.13C has about the same gyromagnetic ratio as .sup.129Xe, so it follows that a similar concentration of .sup.13CO.sub.2 molecules, also polarized to 18%, can have a similar magnetization.

[0029] The CO.sub.2 molecule is weakly diamagnetic, with a molecular magnetic moment per unit volume M/V0.03 T/m3. This leads to the surprising insight that, for sufficient nuclear polarizations, the overall magnetic moment of an ensemble of .sup.13CO.sub.2 molecules willtemporarilybe paramagnetic as the paramagnetic nuclear magnetic moment of the .sup.13C nuclei exceeds the diamagnetic magnetic moment of the CO.sub.2 molecule itself. If the feedstock CO.sub.2 is in the gaseous or liquid state, .sup.13CO.sub.2 molecules (i.e., those CO.sub.2 molecules whose carbon atom is .sup.13C) will therefore be attracted towards field center of a magnet whose vector magnetic field is in the same direction as that of the .sup.13C nuclear polarization. .sup.12CO.sub.2 molecules will remain weakly diamagnetic as the .sup.12C nucleus has a zero magnetic moment and will therefore be weakly repelled from the same field. Thus, in liquid or gaseous CO.sub.2, separation of molecules containing .sup.13CO.sub.2 and .sup.12CO.sub.2 may be achieved by using a magnet to drive polarized .sup.13CO.sub.2 molecules in the opposite direction than unpolarized .sup.12CO.sub.2 molecules.

[0030] In a preferred embodiment, isotopic separation is carried out in liquid CO.sub.2. This is because the relaxation rate of the nuclear polarization is much faster in gas than in liquid CO.sub.2. Relaxation rates in liquid CO.sub.2 have been observed to be 20 seconds, but much less than one second in gaseous CO.sub.2.

[0031] In a preferred embodiment, .sup.13CO.sub.2 molecules, being 1.1% of solid CO.sub.2available in bulk as dry iceare hyperpolarized. by exposing them to a temperature T and a magnetic field B such that the .sup.13C nuclear polarization is much larger than equilibrium. In a preferred embodiment, T is less than 4 Kelvin and B is greater than 10 Tesla. To hasten polarization under conditions of high B/T, the dry ice can be milled to be in a powder form where the average particulate size is <5 microns in diameter. Optionally, various adulterants may be added to the frozen CO.sub.2 powder; these include but are not limited to radicals, paramagnetic nanoparticles, frozen oxygen and other materials known to hasten polarization in a high B/T environment. In an additional embodiment, a Quantum Relaxation Switch (QRS) consisting of .sup.3He and optionally .sup.4He can be used; the process for using QRS to hyperpolarize gasses in a brute force high B/T environment is described in U.S. Pat. No. 6,651,459.

[0032] As can be seen in FIG. 1 (brute force (high B/T) polarization of .sup.13C), exposing .sup.13CO.sub.2 to an environment of 15 Tesla and 150 mK will produce 5% polarization in the .sup.13C nuclei contained in the raw feedstock CO.sub.2 dry ice. Note that very large amounts of material can be cooled to these B/T conditions in relative short order. For example it has been shown in the art that 35 kg of copper can be cooled to 30 mK in 28 hours starting from room temperature. Assuming an additional 20 hours to polarize the .sup.13CO.sub.2, this amounts to 35 kg of raw feed every 48 hours, or 0.35 kg of .sup.13CO.sub.2 every 48 hours. These production rates can potentially meet or exceed current .sup.13CO separation processes via fractional distillation.

[0033] Other methods to polarize molecules containing .sup.13C are known in the art. These include dynamic nuclear polarization (DNP), low field thermal mixing (LFTM), Paharahydrogen Induced Polarization (PHIP). It will be understood that this list is not exhaustive and that the isotopic enrichment method described herein can be used with any combination of hyperpolarization methods.

[0034] Once the .sup.13C nuclei are well polarized, a process which can be optionally monitored via NMR, the feedstock dry ice powder is warmed quickly to 215 K under 5 bar of external gas pressure. The external gas pressure can be provided, for example, by maintaining a pressure of nitrogen gas (or an inert gas or substantially unreactive gas) over the dry ice powder. Under such conditions CO.sub.2 transitions directly from the frozen solid into the liquid state without ever becoming a gas. In a preferred embodiment, this is done in the presence of a large magnetic field, preferably in excess of 1 Tesla, even more preferably in excess of 10 Tesla. The relaxation rate of the .sup.13C nuclear polarization in liquid CO.sub.2 is 20 seconds in a 1 Tesla field. This is more than enough time to melt the powderized CO.sub.2 and carry out isotopic separation described below. The magnetic field can be, for example, anywhere between about one Tesla and about 30 Tesla in increments of about 500 Gauss.

[0035] Magnetic Energy of Liquid Polarized .sup.13CO.sub.2 in a Magnetic Field B:

[0036] The process described above produces a volume containing liquid CO.sub.2 where some percentage of the CO.sub.2 molecules is polarized liquid .sup.13CO.sub.2. A volume of polarized liquid .sup.13CO.sub.2 in an external magnetic field B the magnetic energy per unit mole is

E.sub.mole.sub.0(M.sub.vol)B/

[0037] Where .sub.0 is the permeability constant, B the size of the external magnetic field in Tesla and is the molar density of liquid CO.sub.2. E.sub.mole will vary linearly with .sup.13C polarization and magnetic field B; as an example E.sub.mole is 638 Joules/mole for a .sup.13C polarization of 5% and a magnetic field of 10 Tesla.

[0038] Entropy of Mixing Per Unit Mole:

[0039] As noted above, CO.sub.2 is a liquid at T220 K and ambient pressure of 5 bar. In feedstock liquid CO.sub.2, the .sup.13CO.sub.2 molecules and .sup.12CO.sub.2 molecules can be expected to be completely intermixed. In order to separate liquid .sup.13CO.sub.2 from liquid .sup.12CO.sub.2 the entropy of mixing must be overcome.

[0040] Assume the process begins with a container of liquid CO.sub.2 that fills a volume Vi. To temporarily constrain polarized liquid .sup.13CO.sub.2 to only a portion of that container Vf decreases the entropy of the .sup.13CO.sub.2 molecules.

S.sub.mole=RT ln(V.sub.f/V.sub.i)

Where R is Boltzmann's constant, T is the ambient temperature and Vf, Vi are the final and initial volumes of the liquid .sup.13CO.sub.2, respectively (FIG. 2).

[0041] The entropy of mixing must be overcome to effect isotopic separation. It is clear from the above that this can be done given some combination of a) sufficiently polarized .sup.13CO.sub.2 and b) large enough magnetic field. As a non exhaustive example, the .sup.13C polarization is 5% and the magnetic field is 10 Tesla. In this case the average magnetic energy per mole in a 10 T magnet for the .sup.13CO.sub.2 molecules is 638 J/mole. Thus the minimum Vf/Vi ratio achievable in LCO.sub.2 at 220 K is 0.7. In this scenario the separation constant of the new approach 1.3 and may require 17 separation stages and 1,500 moles of raw feedstock to produce 1 mole of 99% enriched .sup.13CO.sub.2. This separation constant in this non exclusive example is much improved compared to FD of CO which is 1.012 at T70 K and requires 11,400 moles of feedstock CO to produce 1 mole of 99% enriched .sup.13CO.

[0042] The separation constant depends on the .sup.13C polarization. As noted above polarization is not constant, but will decay with a time constant T1. It is important therefore that separation take place as quickly as possible. The rate of separation can be hastened by the use of a large magnetic field gradient over the liquid CO.sub.2 containing some percentage of liquid, polarized .sup.13CO.sub.2 molecules. The attractive magnetic force on a .sup.13CO.sub.2 molecule will be

F/V=(1/.sub.0)*(M)*B

where B is the local magnetic field gradient. .sup.12CO.sub.2 molecules are weakly repelled by the same local field gradient.

[0043] Preferably, the optional field gradient is 10 T/meter, even more preferably 200 T/m. For example, the field gradient can be between about 1 T/meter and about 500 T/meter, or in any increment therebetween of about 1 T/m, or any subrange between said endpoints of about 1 and about 500 T/m that is between about 1 T/m and about SOT/M in magnitude. Such a gradient can be produced for example near the outer edge of a commercially available 20 Tesla superconducting solenoid. In an alternate embodiment, very large local magnetic field gradients can be produced using a combination of steel mesh and an ambient magnetic field. Local field gradients produced by a metallic mesh, such as a magnetized steel mesh are known to be extremely large, on the order of hundreds of T/m. The .sup.13CO.sub.2 molecules are attracted into the steel mesh where they can then be separately collected.

[0044] If a solenoid is used to produce the field gradient, the gradient can optionally be pulsed in time. For example, the gradient can vary from zero to a desired level as set forth in the preceding paragraph, or can vary between a base level and a peak level. For example, the value of the gradient can vary between 0 and 500 T/m, or any subrange therein of about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, or more T/m. The pulses of the gradient can have a peak to peak time, for example, between 1.0 microseconds to about one minute, or any value therein in increments of one microsecond.

[0045] Once some degree of separation between .sup.13CO.sub.2 and .sup.12CO.sub.2 molecules has been achievedthat is, a region of space containing an enriched concentration of .sup.13CO.sub.2 has been establisheda variety of methods can then be used to harvest isotopically rich .sup.13CO.sub.2 molecules from the feedstock CO.sub.2 liquid. For example, the liquid at the top and bottom of the volume in FIG. 1 can be drained or pumped away, with the portion of liquid containing a relatively rich concentration of .sup.13CO.sub.2 being directed to a separate container. In a preferred embodiment, the liquid CO.sub.2 is resolidified once a region of relatively rich concentration of .sup.13CO.sub.2 has been established. This can be achieved by recooling the container to a temperature <200 K, increasing the pressure head above the container, or both. The frozen CO.sub.2 can then be sublimated directly to gas by reducing the pressure head. If heat is applied at the top of the container the gas that comes off the frozen CO.sub.2 first will be relatively poor in .sup.13CO.sub.2; this gas is directed to one container. Subsequently the region containing relatively rich .sup.13CO.sub.2 can be sublimated with that gas directed to a second container. Any of these harvesting steps can be repeated as many times as necessary to achieve a desired level of isotopic purity.

[0046] The above process is described for producing enriched .sup.13CO.sub.2 molecules, which can be used as raw stock to produce a variety of .sup.13C enriched molecules. Similar embodiments can be envisioned for other carbon bearing molecules such as carbon monoxide (CO), methane (CH.sub.4) or other elements with isotopes that differ in their nuclear magnetic moments including, but not limited to: xenon, nitrogen, helium, and others.

[0047] The methods and systems of the present disclosure, as described above and shown in the drawings, provide for isotopic enrichment of various elements/molecules with superior production rates to present techniques. It will be apparent to those skilled in the art that various modifications and variations can be made in the devices and methods of the present disclosure without departing from the spirit or scope of the disclosure. Thus, it is intended that the present disclosure include modifications and variations that are within the scope of the subject disclosure and equivalents.