Patent Application
20180238618
The present invention relates to a process for producing helium from a source gas stream comprising at least helium, methane, nitrogen and hydrogen.
Helium is obtained commercially virtually exclusively from a mixture of volatile components of natural gas, this mixture comprising, along with helium, typically methane and nitrogen and traces of hydrogen, argon and other noble gases. In the course of the production of mineral oil, helium is provided as a component of the gas which accompanies the mineral oil, or in the context of the production of natural gas. It is theoretically possible to obtain helium from the atmosphere, but this is not economical on account of the low concentrations (typical concentration of helium in air of about 5.2 ppmv).
In order to avoid undesirable freezing during a process of liquefaction of helium, the concentration of the impurities in the helium stream to be liquefied must not exceed a value of 1000 ppm by volume, preferably 10 ppmv.
For this reason, the helium liquefaction process is connected downstream of a helium purification process. This is generally composed of a combination of cryogenic processes, generally based on partial condensation, and of adsorption processes, regeneration in the latter case being possible by means of varying the temperature and/or the pressure.
In many cases, it is advantageous to perform a helium purification process such that, in addition to the purified helium, nitrogen of required purity—in which the sum of the impurities is less than 1% by volume—may be obtained. In general, only a portion, typically from 5% to 70%, preferably from 10% to 50%, of the nitrogen present in the mixture to be purified is brought to the desired purity.
The remaining nitrogen is released to the atmosphere at the same time as methane in low-pressure gas form, either directly or after an oxidation step, preferably performed in a torch or an incinerator.
A known example of a prior art process for obtaining a fraction of pure helium from a starting fraction comprising at least helium, methane and nitrogen is described in patent application AU 2013/200 075.
This process for obtaining a fraction of pure helium from a starting fraction comprising at least helium, methane and nitrogen comprises the following successive steps:
a) the starting fraction is subjected to a removal of methane and nitrogen,
b) the fraction obtained from a) which is composed essentially of helium and nitrogen is compressed,
c) the compressed fraction is subjected to a removal of nitrogen, and
d) the helium-rich fraction obtained in step c) is subjected to purification by adsorption to produce a fraction.
Early removal of the methane fraction contained in the initial gas stream to be treated imposes the use of two necessary independent cryogenic steps, and the investment and running costs are thus substantial.
Moreover, part of the nitrogen contained in the initial gas stream to be treated is lost with the ethane removed in the first step. Now, the recycling of nitrogen for other applications is a key element on an industrial scale, since nitrogen, in particular liquid nitrogen, is highly economically upgradable.
In addition, this process does not make it possible to treat gas streams containing a high content of hydrogen, typically more than 6% by volume of hydrogen.
Another type of helium purification process known from the prior art is illustrated by
A gas stream 1′ comprising nitrogen, methane, helium and hydrogen, for example originating from the outlet of a nitrogen rejection unit (NRU) 15′ following the treatment of a natural gas stream to remove the nitrogen from this natural gas, is introduced into a compressor 2′. Once this gas has been compressed, it is introduced into a helium-concentrating device 3′.
At the outlet of this device 3′, the hydrogen contained in the gas stream is removed by means of a system 4′ in which hydrogen and oxygen react.
On conclusion of this step, the gas stream is then purified by means 5′ of a pressure swing adsorption (PSA) process. A gas stream 6′, originating from the PSA 6′, predominantly containing helium is then liquefied in a helium liquefaction device 7′. The liquefied helium is sent to a helium storage system 8′. Said storage system 8′ is cooled with liquid nitrogen 9′ obtained from a liquid nitrogen storage device 10′ fed by an air-separating unit 11′.
Moreover, the liquid nitrogen stored in the device 10′ serves to feed the helium-concentrating device 3′.
The gas stream 12′ containing a majority of nitrogen and a small amount of helium is purified by means of a purification means 13′ which removes the impurities contained in the gas stream 12′ so as to produce a recycling gas stream 14′ sent to the compressor 2′ after having been mixed with the initial gas stream 1′ to be treated.
When the hydrogen content is high, typically more than 4% by volume or even 6%, the input of air into the hydrogen removal system 4′ in which hydrogen and oxygen react is substantial. A large amount of nitrogen and argon is then introduced therein, which dimensions the PSA system 5′.
A purge used in the helium concentrator 3′ contains methane. It must therefore be treated by means of a methane oxidation device to meet the environmental requirements.
It is necessary to have an air-separating unit (ASU) 11′ which produces liquid nitrogen to the specification compatible with the helium storages 8′ (of the order of one ppm of methane).
The inventors of the present invention thus developed a solution for solving the problems raised above.
One subject of the present invention is a process for producing helium from a source gas stream comprising at least helium, methane, nitrogen and hydrogen, comprising at least the following successive steps:
step a): introducing said source gas stream into at least one compressor;
step b): removing hydrogen and methane by reaction of the stream 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 obtained from step c) so as to produce a liquid nitrogen stream and a gas stream predominantly comprising helium;
step e): purifying the gas stream 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 obtained from step d).
According to other embodiments, a subject of the present invention is: A process as defined previously, characterized in that the source gas stream 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, preferably from 5% by volume to 10% by volume of hydrogen.
A process as defined previously, characterized in that the source gas stream 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, preferably from 5% by volume to 10% by volume of hydrogen.
A process as defined previously, comprising a step prior to step a) of producing the source gas stream to be treated by means of a nitrogen rejection unit or a natural gas liquefaction unit, said unit producing a liquid nitrogen stream used in step d) allowing partial condensation of the stream obtained from step c) so as to produce a liquid nitrogen stream and a gas stream predominantly comprising helium.
A process as defined previously, characterized in that the pressure on conclusion of step a) is between 15 bara and 35 bara, preferably between 20 bara and 25 bara.
A process as defined previously, characterized in that the gas stream obtained from step b) comprises less than 1 ppm by volume of hydrogen and less than 1 ppm by volume of methane.
A process as defined previously, characterized in that said impurities contained in the gas stream obtained from step b) predominantly comprise carbon dioxide and water.
A process as defined previously, characterized in that the liquid nitrogen stream obtained from step d) comprises more than 98.5% by volume of nitrogen.
A process as defined previously, characterized in that said gas stream obtained from step d) comprises between 80% by volume and 95% by volume of helium.
A process as defined previously, characterized in that said gas stream obtained from step e) comprises at least 99.9% by volume of helium.
A process as defined previously, characterized in that step b) consists in placing the gas stream obtained from step a) 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, supported on a support that is chemically inert with respect to carbon dioxide and water so as to react the methane and hydrogen with oxygen.
A process as defined previously, characterized in that it comprises an additional step f) of liquefaction of the helium obtained from step e).
A process as defined previously, characterized in that the liquid nitrogen derived from step d) cools the helium liquefied in step f).
An installation for producing helium from a source gas mixture comprising methane, helium, hydrogen and nitrogen, comprising at least one compressor directly receiving the source gas mixture, at least one means for removing hydrogen and methane, at least one nitrogen-removing and helium-concentrating device, and at least one helium purification means located downstream of the nitrogen-removing and helium-concentrating device, characterized in that the means for removing hydrogen and methane is located downstream of said at least one compressor and upstream of the nitrogen-removing and helium-concentrating device.
An installation as defined previously, characterized in that it also comprises a helium liquefaction device downstream of the helium purification means.
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:
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.
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.
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.
This catalyst is typically Pd/Al2O3. Catalytic oxidation between oxygen and the combustives (hydrogen/methane) takes place.
The hydrogen reacts with the oxygen to form water. Since this reaction is exothermic, the temperature rises.
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).
Thus, the hydrogen and methane contained in the initial source stream 1 to be treated are oxidized with oxygen from the unit 5.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
Advantages of a process as illustrated in
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).
The purge originating from the cryogenic helium-concentrating unit 9 no longer contains any methane (which has been oxidized beforehand).
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).
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.
For example, the stream 21 comprises between 40% and 50% by volume of helium and between 50% and 60% by volume of nitrogen.
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
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
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).
The table below summarizes the compositions of the gas streams entering the helium purification unit (element numbered 13 in
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1553906 | Apr 2015 | FR | national |
This application is a 371 of International PCT Application PCT/FR2015/052633, filed Oct. 1, 2015, which claims priority to French Patent Application No. 1553906, filed Apr. 30, 2015, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2015/052633 | 10/1/2015 | WO | 00 |
