PRODUCTION OF HELIUM FROM A GAS STREAM CONTAINING HYDROGEN

Abstract

The invention relates to a method for producing helium from a source gas stream (1) including at least helium, methane, nitrogen and hydrogen, comprising at least the following consecutive steps: step a): injecting said source gas stream (1) into at least one compressor (3); step b): eliminating the hydrogen and the methane by reacting the stream (4) obtained from step a) with oxygen; step c): eliminating at least the impurities from step b) by temperature swing adsorption (TSA); step d): partially condensing the stream (8) obtained from step c) in order to produce a stream (10) of liquid nitrogen and a gas stream (11) comprising mostly helium; step e): purifying the gas stream (11) obtained from step d) in order to increase the helium content by pressure swing adsorption (PSA) by eliminating the nitrogen and the impurities contained in the gas stream (11) obtained from step d).

Claims

1. A process for producing helium from a source gas stream (1) comprising at least helium, methane, nitrogen and hydrogen, the process comprising at least the following successive steps: step a): introducing said source gas stream (1) into at least one compressor (3); step b): removing hydrogen and methane by reaction of the stream (4) obtained from step a) with oxygen; step c): removing at least the impurities obtained from step b) by temperature swing adsorption (TSA); step d): partially condensing the stream (8) obtained from step c) so as to produce a liquid nitrogen stream (10) and a gas stream (11) predominantly comprising helium; step e): purifying the gas stream (11) obtained from step d) so as to increase the helium content by pressure swing adsorption (PSA) by removing the nitrogen and the impurities contained in the gas stream (11) obtained from step d).

2. The process of claim 1, wherein the source gas stream (1) comprises from 40% to 95% by volume of nitrogen, from 0.05% to 40% by volume of helium, from 50 ppmv to 5% by volume of methane and from 1% to 10% by volume of hydrogen.

3. The process as claimed in claim 2, characterized in that the source gas stream (1) comprises from 40% to 60% by volume of nitrogen, from 30% to 50% by volume of helium, from 50 ppmv to 5% by volume of methane and from 1% to 10% by volume of hydrogen.

4. The process of claim 1 further comprising a step prior to step a) of producing the source gas stream (1) to be treated by means of a nitrogen rejection unit (2) or a natural gas liquefaction unit, said nitrogen rejection unit (2) or natural gas liquefaction unit producing a liquid nitrogen stream (20) used in step d) to partially condense the stream (8) obtained from step c).

5. The process of claim 1, wherein the pressure of the source gas stream (1) on conclusion of step a) is between 15 bara and 35 bara.

6. The process of claim 1, wherein the gas stream (6) obtained from step b) comprises less than 1 ppm by volume of hydrogen and less than 1 ppm by volume of methane.

7. The process of claim 1, wherein said impurities contained in the gas stream (6) obtained from step b) predominantly comprise carbon dioxide and water.

8. The process of claim 1, wherein the liquid nitrogen stream obtained from step d) comprises more than 98.5% by volume of nitrogen.

9. The process of claim 1, wherein said gas stream obtained from step d) comprises between 80% by volume and 95% by volume of helium.

10. The process of claim 1, wherein said gas stream obtained from step e) comprises at least 99.9% by volume of helium.

11. The process of claim 1, wherein in step b) the gas stream obtained from step a) is placed in contact with oxygen and a catalytic bed comprising particles of at least one metal chosen from copper, platinum, palladium, osmium, iridium, ruthenium and rhodium, wherein the metal is supported on a support that is chemically inert with respect to carbon dioxide and water, and wherein to the catalyst bed catalyzes a reaction of the methane and hydrogen with oxygen.

12. The process of claim 1, further comprising an additional step f) of liquefaction of the helium obtained from step e).

13. The process of claim 1, wherein in the liquid nitrogen obtained from step d) cools the helium liquefied in step f).

14. An installation for producing helium from a source gas mixture (1) comprising methane, helium, hydrogen and nitrogen, comprising at least one compressor (3) directly receiving the source gas mixture (1), at least system (5) for removing hydrogen and methane, at least one nitrogen-removing and helium-concentrating device (9), and at least one helium purification system (13) located downstream of the nitrogen-removing and helium-concentrating device (9), wherein the system (5) for removing hydrogen and methane is located downstream of said at least one compressor (3) and upstream of the nitrogen-removing and helium-concentrating device (9).

15. The installation of claim 14, further comprising a helium liquefaction device (17) downstream of the helium purification means (13).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0048] FIG. 1 illustrates a block flow diagram of a state of the art helium purification plant for separating helium from a nitrogen rejection system for natural gas purification; and

[0049] FIG. 2 illustrates a block flow diagram of an embodiment of the invented helium purification plant for separating helium from a nitrogen rejection system for natural gas purification.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0050] A source gas stream 1 containing at least helium, nitrogen, hydrogen and methane is treated via a process that is the subject of the present invention so as to produce a pure helium stream, typically containing more than 99.999% by volume of helium. The source stream 1 originates, for example, from a nitrogen rejection unit (NRU) 2 located downstream of a cryogenic unit for treating natural gas.

[0051] The source stream 1 is introduced into a compressor 3 allowing the gas stream 4 to be compressed to a pressure of between 15 bara (bar absolute) and 35 bara, preferably between 20 bara and 25 bara. The temperature is the ambient temperature at the site of the installation.

[0052] The gas stream 4 is introduced into a unit 5 for removing hydrogen and methane. This unit 5 consists, for example, of one or more reactors in series containing a catalyst between grilles.

[0053] This catalyst is typically Pd/Al.sub.2O.sub.3. Catalytic oxidation between oxygen and the combustives (hydrogen/methane) takes place.

[0054] The hydrogen reacts with the oxygen to form water. Since this reaction is exothermic, the temperature rises.

[0055] To oxidize the methane also, higher temperatures are required. A high content of hydrogen at the inlet makes it possible to work at a high temperature and to co-oxidize the methane (for example, with 2% of hydrogen, the temperature rises to about 200 C., which is not sufficient to oxidize methane).

[0056] Thus, the hydrogen and methane contained in the initial source stream 1 to be treated are oxidized with oxygen from the unit 5.

[0057] Impurities such as water and carbon dioxide are thus produced in the gas stream 6 leaving the unit 5. This gas stream 6 predominantly comprises nitrogen and helium.

[0058] The exiting gas is cooled (against the ambient air or cooling water) before being sent to the adsorption unit 7. Some of the water then condenses directly in a condensate recuperator. Some of the heat produced may be recovered to be used in another process.

[0059] The gas stream 6 is then treated in an adsorption unit 7, such as a temperature swing adsorption (TSA) unit, so as to remove the water and carbon dioxide from the gas stream 6. This results in a gas stream 8 essentially comprising nitrogen and helium (i.e. comprising less than 5 ppm by volume of methane, less than 1 ppm by volume of hydrogen, less than 0.1 ppm by volume of carbon dioxide and less than 0.1 ppm by volume of water). The gas stream 8 is treated in a nitrogen-purifying and helium-concentrating unit 9.

[0060] This unit 9 comprises at least one heat exchanger in which the gas stream is cooled from the ambient temperature (0 C.40 C., for example) to a temperature of between 180 C. and 195 C. On leaving this heat exchanger, the gas stream is introduced, for example, into a phase-separating pot generating a liquid stream 10 and a gas stream 11.

[0061] The liquid stream 10 contains 98.8% by volume of nitrogen. This liquid stream 10 is sent to a liquid nitrogen storage device 12. It does not contain any methane.

[0062] The gas stream 11 contains from 80% by volume to 95% by volume of helium and from 5% by volume to 20% by volume of nitrogen. The stream 11 is sent to a helium purification unit 13.

[0063] This purification unit 13 is, for example, a pressure swing adsorption (PSA) unit and produces two streams. One stream, 14, contains 99.9% by volume of helium and another stream, 15, contains the rest of the elements (essentially nitrogen). The gas stream 15 is introduced into a compressor 16 and then mixed with the source gas stream 1 to be treated; this is a regeneration loop of the unit 13.

[0064] The helium-rich stream 14 may be sent to a helium liquefaction unit 17 producing a liquid helium stream 18 conveyed to a storage device 19. The pure liquid nitrogen 10 stored in the nitrogen storage device 12 may be used to maintain the temperature of the helium storage device 19.

[0065] According to a preferred embodiment, a liquid nitrogen stream 20 produced by the nitrogen rejection unit 2 is introduced into the nitrogen-purifying and helium-concentrating unit 9. This makes it possible to obtain the cooling power required and to thereby avoid investment in a dedicated air-separating unit, in contrast with the process illustrated in FIG. 1.

[0066] Use may also be made of another cold-generating fluid present on site (for example LNG) or of a high-pressure fluid which is expanded (via joule Thomson expansion or turbines) to create the required refrigeration.

[0067] Advantages of a process as illustrated in FIG. 2 that is the subject of the present invention relative to the process illustrated in FIG. 1 are described below.

[0068] Simultaneous oxidation of hydrogen and methane takes place before helium concentration. The TSA 7 then functions under pressure, which ensures better efficiency (reduction of the required volume of adsorbents and also reduction of the heat consumption in the regeneration reheater).

[0069] The purge originating from the cryogenic helium-concentrating unit 9 no longer contains any methane (which has been oxidized beforehand).

[0070] Methane-free liquid nitrogen 10 may thus be produced from the unit 9. It suffices to integrate this unit 9 with the helium-concentrating unit 2 (NRU or natural gas liquefaction unit) to obtain the required cooling power. This makes it possible to avoid investment in a dedicated air-separating unit (ASU).

[0071] According to a particular mode of the invention, a stream 21 expanded beforehand in the unit 9 containing nitrogen and helium is extracted from said unit 9 and then sent to a compressor 3 and/or 16. Thus, helium obtained from the expansion of the liquid nitrogen from the unit 9 is recycled so as to increase the percentage of helium produced.

[0072] For example, the stream 21 comprises between 40% and 50% by volume of helium and between 50% and 60% by volume of nitrogen.

[0073] The yield of the PSA unit 13 and its size are also greatly improved. The helium 11 is preconcentrated to about 90% in the PSA 13 (rather than 70% in the process of FIG. 1 and with a high content of hydrogen. The argon and oxygen impurities are also in a much lower amount (since the argon and oxygen condense out at the same time as the nitrogen).

[0074] There is also no more carbon dioxide or water to be treated in the entering gas. The pressure of the residual gas (offgas) of the PSA 13 may also be reduced relative to that of the process illustrated in FIG. 1 since they can return directly to the compressor 16 without passing beforehand through a drying unit.

[0075] All these points make it possible to improve the yield of the PSA 13, which dimensions the return line and the compressor 3 of the stream 1 to be treated (the energy consumption of the compressor is reduced).

[0076] The table below summarizes the compositions of the gas streams entering the helium purification unit (element numbered 13 in FIGS. 2 and 5 in FIG. 1).

TABLE-US-00001 TABLE Composition of the gases entering the PSA Gas stream Composition FIG. 1 FIG. 2 He mol % 69.48% 89.9697% N.sub.2 mol % 29.94% 9.9979% CH.sub.4 ppmv 1 1 Ar ppmv 2658 181 H.sub.2 ppmv <0.5 <0.5 Ne ppmv 300 300 CO ppmv 0 0 O.sub.2 ppmv 2703 143 H.sub.2O saturated 0 CO.sub.2 ppmv 355 <0.1 Total mol % 100% 100% Flow rate (sec) Nm.sup.3/h 4806 3713 Pressure bara 23.55 23.45 Temperature C. 47 47

[0077] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.