GB2360574A

GB2360574A - Storing a gas by encapsulation, particularly in an adsorbent.

Info

Publication number: GB2360574A
Application number: GB0007214A
Authority: GB
Inventors: Roy Clampitt
Original assignee: Oxford Applied Research Ltd
Current assignee: Oxford Applied Research Ltd
Priority date: 2000-03-25
Filing date: 2000-03-25
Publication date: 2001-09-26
Also published as: US20010037647A1; US6516619B2; GB0007214D0; EP1139006A1

Abstract

A method of storing a gas comprises condensing a gas, in one example by cryosorbing the gas into an absorbent medium. The cryosorbed gas is then encapsulated. Encapsulating the cryosorbed gas increases the temperature at which the absorbent medium can be kept before the cryosorbed gas is released. The encapsulation layer can be provided by freezing a material that has a lower vapour pressure than the gas. One suitable material for forming the encapsulation layer in this way is water. This method can be used to store gases having a very low boiling point, such as hydrogen or oxygen. This method does not require that the gas is condensed into an absorbent storage medium. The method can also be used to encapsulate, for example, pellets of frozen gas.

Description

2360574 A method of storing a gas The present invention relates to a

method of storing a gas, in particular to a method of storing a gas that has a freezing point that is substantially below O'C.

The tertn "gas" as used in this specification refers to a substance that is gaseous under conditions of room temperature and atmospheric pressure.

There is currently considerable interest in replacing petrol and related petrochemical fuels with fuels that produce less pollution. Substances that are being considered for use as an alternative fuel for, for example, vehicle propulsion include methanol and hydrogen. This worldwide research activity in alternative fuels is generally known as "the hydrogen economy".

One factor that is important in the commercial acceptance of an alternative fuel is the ease of storing the fuel. Petrol is a liquid at normal ambient temperatures, and so is easy to store and transport. Of the two alternative fuels mentioned above. methanol is a liquid at room temperature (melting point -98'C, boiling point WC), but hydrogen is a gas at room temperature (melting point -259'C, boiling point -252'C). In order for hydrogen to be widely used as a fuel, one problem that must be overcome is to find a convenient way of storing hydrogen. Various methods of storing hydrogen are currently being explored, such as storing hydrogen as a liquid, as a high-pressure gas, as a metal hydride, and storing by cryosorption. The present invention relates to a cryosorption method of storing hydrogen or other gases.

It is well known that hydrogen, and other gases, can be physisorbed into the microscopic pores of materials such as zeolites (alumino-silicates) and "activated" carbon. Materials suitable for physisorbing gases are commercially available. The physisorption can be achieved at room temperature by high pressure absorption, but it can more effectively be carried out by physisorption at cryogenic temperatures.

2 The capacity of a material to absorb a gas can be increased by the addition of a catalytic metal, such as palladium or platinum. The phenomenon of this increase in absorption capacity when a catalyst is added is known as chemisorption, and it is believed that a gas, such as molecular hydrogen, is dissociatively absorbed within the catalystimpregnated absorbent material.

The use of carbon to store hydrogen cryogenically in this manner is disclosed in, for example, US Patent No. 4 716 736, by Amankwah et al in "Int J of Hydrogen Energy" Vol 14, pp 437 (1989), and by Carpetis et al in "Int J of Hydrogen Energy" Vol 5, pp 539 (1980). The temperature at which storage is found to be practicable is variously reported to be between 4'K and 15WK (O'C = 273'K), and the stored hydrogen is released for use by a controlled increase in the temperature of the absorbent material.

It will be appreciated that this method of storing hydrogen requires that the absorbent material is kept at a temperature of 1501K (- 1 23'C) or below, and it is expensive and inconvenient to do this. In order to make a cryosorption method of storing gas more economically attractive it is desirable to raise the temperature at which the hydrogen can be stored.

In the early development of vacuum pumps it was discovered that materials such as the above-mentioned zeolites and activated carbon could aid the evacuation of air from a vessel by the phenomenon of "cryopumping". Pumps that operate on the basis of this phenomenon are now commercially available, and are known as "cryopumps". It was noted during these early developments that when a vapour such as water is condensed it is possible that "permanent" gases could be trapped within the condensed vapour. (A 44perTnanent" gas has a boiling point substantially below O'C.) This phenomenon is known as cryotrapping, and is reported by, for example, Schmidlin in "Transactions of 9th American Vacuum Symposium" pp 197 (1962); MaeMillan NY. The cryotrapping phenomenon is not limited to the entrapment of hydrogen - for example, Hengevoss et al have reported, in "Transactions of 1 Oth American Vacuum Symposium" pp 10 1 (1963) MaeMillan NY, the cryotrapping of argon in condensed water vapour. The present inventors have also demonstrated cryotrapping of hydrogen and argon 3 simultaneously when water vapour is condensed as ice under vacuum conditions. It is possible to show that a gas has been cryotrapped in, for example, condensed water vapour by detecting the subsequent release of the entrapped gas upon the heating of the condensed water vapour to a temperature, typically 160-200'K, at which the water remains frozen. This can be done, for example, using a mass spectrometer. The phenomenon of "cryotrapping" is now well known in the field of vacuum technology.

In the cryotrapping phenomena described above, the hydrogen or argon is trapped in the condensed water vapour during the process of condensing the water vapour. Water has a lower vapour pressure than either hydrogen or argon, so that the pores of the storage medium will be preferentially filled with water-ice rather than with hydrogen or argon. The capacity of the storage medium to store hydrogen or argon is thus severely restricted. This method is therefore unsuitable for the large-scale storage of gases such as hydrogen.

A first aspect of the present invention provides a method of storing a gas comprising the steps of. condensing a gas; and encapsulating the condensed gas by freezing an encapsulant material onto the condensed gas.

Encapsulating the condensed gas increases the maximum temperature at which the condensed gas can be kept without release of the gas.

The gas may be condensed intolonto an absorbent storage medium. In this embodiment the gas is condensed into the absorbent medium in the absence of the encapsulant material, so that the encapsulant material does not compete with the gas to fill the pores of the absorbent storage medium. In consequence the pores of the absorbent medium are filled with the gas to be stored, not with the encapsulant material, and the capacity of the absorbent medium to store the gas is increased significantly, probably by orders of magnitude, compared to a conventional co-condensation cryotrapping method.

A second aspect of the present invention provides a method of storing and releasing a gas comprising the steps of. storing a gas by a method as defined above; and increasing 4 the temperature of the stored gas to a temperature at which stored gas is vaporised and released through the frozen encapsulant material.

A third aspect of the present invention provides a stored gas comprising: a condensed gas; and an encapsulant layer encapsulating the condensed gas, the encapsulant layer having been formed by freezing an encapsulant material.

Preferred features of the present invention are set out in the dependent claims.

Preferred embodiments of the present invention will now be described by way of illustrative example with reference to the accompanying figures in which:

Figure 1 is a schematic view of an apparatus suitable for performing the method of the present invention.

In the present invention, the method of storing a gas has two steps. Initially, the gas to be stored is condensed. In a Preferred embodiment, the gas to be stored is condensed and absorbed into an absorbent storage material, such as activated carbon, a zeolite, or other suitable absorbent material. This is done by cryosorption of the gas into the absorbent material. In the present invention, cryosorption of the gas to be stored is carried out in the absence of a co-condensing gas such as water. Thus, the gas to be stored does not have to compete with a co- condensing gas to fill the pores of the absorbent storage material, and the storage capacity of the absorbent material will be taken up solely by the gas to be stored.

In the absence of any further processing of the absorbent material, when the refrigerant used to perform the cryosorption step were removed the cryosorbed gas would tend to re-evaporate and exert its normal vapour pressure in an enclosed container. For hydrogen, the vapour pressure is one atmosphere at 2WIC, 9.5 atmospheres at 3 1 'K, and many atmospheres at 7WK. Thus, to store commercially useful quantities of, for example, hydrogen in the pores of an absorbent material, the storage temperature would conventionally have to be maintained at a very low level, and this would require expensive reffigeration equipment.

According to this embodiment of the present invention, therefore, after the gas has been stored within the absorbent material the stored gas is encapsulated within the absorbent material. By encapsulating the stored gas, it is possible for the gas to be stored at much higher temperatures, flor example up to around 200'K. Where this method of the invention is used to store hydrogen, even if the temperature of the absorbent material rises to 200'K the cryosorbed hydrogen remains encapsulated and does not reevaporate.

One preferred method of encapsulating the stored gas is to encapsulate by freezing another material to form a capping or sealing layer around the stored gas that has been previously condensed within the pores of the absorbent material. If the stored gas is encapsulated in this way, it is possible to perform the encapsulation in the same apparatus used to cryosorb the gas into the absorbent medium, and this simplifies the process.

The material used to form the encapsulating layer preferably has a higher freezing point than the gas to be stored, so that its vapour pressure at a given temperature is lower than that of the gas to be stored. This ensures that, when the temperature of the absorbent material is increased, the stored gas will evaporate before the encapsulating layer, so ensuring that the released gas is not contaminated by the material of the encapsulating layer.

One suitable material for forming the encapsulation layer is water. It has been found by the inventors that an absorbent medium that contains cryosorbed hydrogen encapsulated by water ice can be stored at temperatures up to 200'K. Moreover the absorbent material can be raised to a temperature which, while still below the melting point of water-ice, allows hydrogen gas to be released through the water-ice layer. Thus, it is possible to obtain hydrogen gas that is not contaminated by water vapour.

6 Thus, the method of the present invention enables a gas that it is desired to store to be cryosorbed into an absorbent material at a temperature of, say, 78'K. This temperature can be readily achieved, by means of liquid nitrogen refligerant. The absorbed gas - for example hydrogen - is encapsulated, for example with a layer of water-ice, and can then be stored at temperatures of up to around 200'K. This temperature is generally within the regime of solid carbon dioxide or household refrigerants, so that it is relatively straightforward to store the gas at this temperature. Furthermore, the absorbent material that has been processed according to the invention may be mechanically or manually manipulated in normal atmosphere or atmospheric surroundings or environment without loss of the stored hydrogen gas, and without danger of fire or explosion, provided that it is kept at or below 200'K.

For subsequent use of the stored gas, for example in a fuel cell, it may be controllably desorbed from the absorbent material simply by raising the temperature of the absorbent material. The temperature of the absorbent material can be chosen to provide a desired vapour pressure of the stored gas. Since the material used to encapsulate the gas has a substantially lower vapour pressure than the stored gas, the vapour pressure of the capping material during the desorption process remains at a level such that it does not contribute to or contaminate the gas stream.

The precise mechanism by which the stored gas is released through the encapsulant or capping layer is not yet fully understood. However, it has been suggested, by R. Scott Smith et al, Thys. Rev. Lett' Vol 79, pp909 (1997), that the release of gas trapped beneath a layer of water-ice, at vacuum monolayer levels, coincides with the transformation of the waterice from an amorphous phase to a crystalline water phase. This transformation occurs at a temperature of around 160'K, and this is close to the observed temperature at which stored gas is released under the inventors' experimental conditions.

The co-condensation/cryotrapping process has been hitherto been carried out under vacuum conditions. In contrast, one feature of the present invention is that the step of cryosorbing the gas to be stored into the absorbent material is preferably carried out at a 7 pressure in excess of atmospheric pressure, and is particularly preferably carried out at a pressure substantially above atmospheric pressure since the efficiency of the absorption process increases with the pressure of the gas.

Figure 1 is a schematic illustration of an apparatus suitable for carrying out the method of the present invention. The apparatus primarily comprises a pressure vessel 1 that is provided with first and second inlet valves 2, 3. The valve will also be provided with other fittings that are conventionally provided in a pressure vessel such as one or more pressure gauges, a temperature measuring instrument such as a thermocouple, and an outlet valve. These fittings are not shown in Figure 1 for clarity.

The first step is to introduce an absorbent material 4 into the pressure vessel 1. 1n one example, the absorbent material is 10g of commercially available activated carbon granules (BDH 330344Y), but any suitable absorbent material can be used. The absorbent material has preferably been treated to remove any absorbed water vapour, for example by heating it.

Once the absorbent material has been introduced in the pressure vessel 1, and the pressure vessel has been sealed, the pressure vessel is then flushed to remove air from the interior of the pressure vessel. The flushing is conveniently carried out by passing hydrogen through the vessel 1 via the valve 2.

Once the vessel has been flushed, the vessel is pressurised with hydrogen gas to a pressure of 4 atmospheres absolute (that is, to a pressure 3 bar in excess of normal atmospheric pressure).

The pressure vessel is then immersed in a liquid nitrogen bath 5. This will cool the pressure vessel 1 and its contents to 78'K. The effect of this cooling is to cryosorb hydrogen gas into the absorbent material 4. The pressure of hydrogen gas within the vessel 1 falls to typically 200300mbar absolute. The pressure following absorption of gas into the absorbent material depends upon (a) the 'fill' pressure and the chamber 8 volume and (b) the quantity of absorbent materiaL In the example cited, equilibrium is attained after approximately 2-4 minutes.

Once the cryosorption process has been completed, the valve 3 is opened to expose the absorbent material 4 to water vapour for around 15-20 minutes. During this time, a capping layer of water-ice is formed over the hydrogen gas that has been cryosorbed in the absorbent medium 4.

Finally, the pressure vessel 1 is removed from the liquid nitrogen bath (or vice versa).

In an experiment that monitors the temperature and pressure within the pressure vessel 1 when it is removed from the liquid nitrogen bath 5, it has been found that the pressure of hydrogen within the pressure vessel 1 remains unchanged until the temperature of the pressure vessel 1 has risen to around 170'K-200'K. At this point, the pressure of hydrogen within the vessel 1 rises rapidly to several bar, and this indicates that the stored hydrogen has been desorbed from the absorbent medium 4. The vapour pressure of hydrogen within the pressure vessel can be controlled by altering the temperature of the vessel, and this enables the hydrogen gas to be supplied at any desired pressure.

It is possible to re-absorb the hydrogen gas into the absorbent medium 4, by immersing the pressure vessel in the liquid nitrogen bath for a second time. It has been found unnecessary to remove the original encapsulation layer, since the gas is apparently cryosorbed through it (or possibly on it). The re-stored hydrogen gas can then be reencapsulated by a further layer of water ice, stored and released controllably when desired.

The absorption capacity of the activated carbon at a charging pressure of 3bar has been found to be typically 14g.H2/kg. carbon. This value is similar to those reported by other workers for activated carbons which also exhibit a linear absorption capacity for hydrogen at pressures up to 50bar (see, for example, Amankawah et al, Int J of Hydrogen Energy" Vol 16, pp3 79 (199 1).

9 The present invention is not limited to the embodiment described above, but can be varied in many ways.

For example, although the example described above uses activated carbon as the absorbent medium, the invention is not limited to the use of activated carbon. Any suitable absorbent medium can be used, including, but not limited to the abovementioned zeolites.

The invention is not limited to the storage of hydrogen gas. In principle, any gas that can be cryosorbed into an absorbent medium can be stored by the method of the present invention. In particular, the present invention can be applied to the storage of oxygen gas.

The material used to foiTn the capping layer is not limited to water-ice. In principle, any substance having a substantially lower vapour pressure than the vapour pressure of the stored gas, and that does not significantly contaminate the gas during its storage or release can be used to form the capping layer.

The present invention has been described with particular reference to encapsulating a gas-charged absorbent medium - that is, to a method in which the gas to be stored is condensed into an absorbent medium before encapsulation. The present invention is not limited to this, but can in principle be used to encapsulate any condensed gas (provided that the vapour pressure of the gas is higher than the vapour pressure of the encapsulant material). For example, the gas to be stored could simply be frozen to form a pellet of soldified gas, and the pellet can then be encapsulated by freezing an encapsulant material to produce an encapsulant layer around the pellet. The formation of solid hydrogen pellets at 41'K is currently being developed for the re-faelling of nuclear fusion reactors, but these must be kept at very low temperatures to prevent them from vaporising. By encapsulating them according to the present invention, the temperature at which they can be kept is raised.

Claims

CLAIMS:

1. A method of storing a gas comprising the steps of.

a) condensing a gas; and b) encapsulating the condensed gas by freezing an encapsulant material onto the condensed gas.

2. A method as claimed in claim 1 wherein step (a) comprises condensing the gas intolonto an absorbent medium.

3. A method as claimed in claim 1 or 2 wherein step (a) comprises cryosorbing the gas into the absorbent medium

4. A method as claimed in claim 3 wherein step (a) comprises cryosorbing the gas into the absorbent medium at a pressure equal to or greater than atmospheric pressure.

5. A method as claimed in claim 3 or 4 wherein step (a) comprises cryosorbing the gas into the absorbent medium at a pressure of substantially 3 bar above atmospheric pressure.

6. A method as claimed in any preceding claim wherein the encapsulant material has a lower vapour pressure than the gas.

7. A method as claimed in any preceding claim wherein the encapsulant material is water.

8. A method as claimed in any preceding claim wherein the gas is hydrogen gas or oxygen gas.

9. A method of storing a gas substantially as described herein above with reference to the accompanying drawing.

10. A method of storing and releasing a gas comprising the steps of..

storing a gas by a method as defined in any of claims 1 to 9; and increasing the temperature of the stored gas to a temperature at which stored gas is vaporised and released through the frozen encapsulant material.

11. A method of storing and releasing a gas as claimed in claim 10 wherein the temperature of the stored gas is selected such that substantially no vaporisation of the frozen encapsulant material occurs when stored gas is vaporised and released.

12. A method of storing and releasing a gas.as claimed in claim 10 or 11 and comprising the step of varying the temperature of the stored gas thereby to vary the vapour pressure of the released gas.

13. A stored gas comprising: a condensed gas; and an encapsulant layer encapsulating the condensed gas, the encapsulant layer having been formed by freezing an encapsulant material.

14. A stored gas as claimed in claim 12 or 13 wherein the absorbed gas is hydrogen gas or oxygen gas.

14. A stored gas as claimed in claim 13 wherein the encapsulant material is water.

15. A stored gas as claimed in claim 13 or 14 wherein the condensed gas is condensed hydrogen gas or condensed oxygen gas.

16. A stored gas as claimed in claim 13, 14 or 15, wherein the condensed gas has been condensed intolonto an absorbent medium.

Amendments to the claims have been filed as follows M&C Folio No P51047GB CLAIMS:

1. A method of storing gas comprising the steps of.

a) absorbing gas onto/into an absorbent medium; and b) encapsulating the gas by freezing an encapsulant material onto the absorbent medium.

2. A method as claimed in claim 1 wherein step (a) comprises cryosorbing gas into the absorbent medium 3. A method as claimed in claim 2 wherein step (a) comprises cryosorbing gas into the absorbent medium at a pressure equal to or greater than atmospheric pressure.

4. A method as claimed in claim 2 or 3 wherein step (a) comprises cryosorbing gas into the absorbent medium at a pressure of substantially 3 bar above atmospheric pressure.

5. -A method as claimed in any preceding claim wherein the encapsulant material has a lower vapour pressure than the gas.

6. A method as claimed in any preceding claim wherein the encapsulant material is water.

7. A method as claimed in any preceding claim wherein the gas is hydrogen gas or oxygen gas.

8. A method of storing a gas substantially as described herein above with reference to the accompanying drawing.

M&C Folio No PSI 047GB 9. A method of storing and releasing a gas comprising the steps of.

storing a gas by a method as defined in any of claims 1 to 8; and increasing the temperature of the stored gas to a temperature at which stored gas is released through the frozen encapsulant material.

10. A method of storing and releasing a gas as claimed in claim 9 wherein the temperature of the stored gas is selected such that substantially no release of the frozen encapsulant material occurs when stored gas is released.

11. A method of storing and releasing a gas as claimed in claim 9 or 10 and comprising the step of varying the temperature of the stored gas thereby to vary the vapour pressure of the released gas.

12. A stored gas comprising: a gas-charged absorbent medium; and an encapsulant layer encapsulating the absorbent medium, the encapsulant layer having been formed by freezing an encapsulant material.

13. A stored gas as claimed in claim 12 wherein the encapsulant material is water.