PRODUCTION OF HELIUM FROM A STREAM OF NATURAL GAS

Abstract

A method for producing a helium gas stream from a gas source containing at least helium, methane, and nitrogen is provided. The method includes introducing the gas into a double column nitrogen rejection unit.

Description

BACKGROUND

The present invention relates to a process for producing helium from a source gas stream comprising at least helium, methane and nitrogen.

Helium is commercially obtained virtually exclusively from a mixture of volatile components of natural gas, this mixture comprising, along with the helium, typically methane and nitrogen and traces of hydrogen, of argon, and of other noble gases.

During the production of mineral oil, helium is made available 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 in the atmosphere, but it is not economical because of the low concentrations (typical concentration of helium in air of about 5.2 ppmv).

Crude natural gas can contain a large number of impeding impurities that need to be removed. Nitrogen is one example of this. Starting from a certain concentration of nitrogen in natural gas, the latter is typically not saleable because of its low calorific value. A cryogenic process carried out in a unit referred to as a nitrogen rejection unit (NRU) is usually used to remove the nitrogen.

If, in addition to the nitrogen, there is helium in the natural gas, there is an economical benefit to extracting a helium-rich stream separately in order to produce helium and to sell it. Typically, the helium is concentrated in the cryogenic process and is finally purified in a PSA (pressure-swing adsorption) process.

Several solutions exist at the current time for extracting helium in an NRU, in particular in a “double-column” NRU unit.

It is possible to add a column for distilling the nitrogen and the helium, as is described in patent EP 0 633 437 B1 for example, but this poses several problems:

• • complexification of the installation;
  • energy consumption increased because of the need for refrigeration (cycle) linked to this column.

Other solutions exist using successive partial condensations and/or instantaneous (flash) vaporizations, but they do not generally make it possible to have both a high helium content and a high yield. One example is described in U.S. Pat. No. 4,701,201. In said patent, the helium is recovered at the top of the high-pressure (HP) column after partial condensation. This simple partial condensation makes it necessary to choose between helium yield and purity. In the patent, it is also indicated that it is possible to achieve a reasonable yield (approximately 92%), but the purity achieved for the helium-enriched stream is 37.2%.

There is thus a need to solve the problems described above.

SUMMARY

For this reason, the subject of the present invention is a process for producing a helium gas stream from a source gas stream comprising at least helium, methane and nitrogen, comprising at least the following steps:

step a): introducing said source gas stream into a double-column nitrogen rejection unit, said double column comprising a high-pressure distillation column, a low-pressure distillation column and a condenser linking the high-pressure column with the low-pressure column;

step b): extracting at the outlet of said condenser at least one part of a mixture produced at the top of the high-pressure column;

step c): expanding said mixture resulting from step b) to an intermediate pressure of between 8 bar and 20 bar absolute;

step d): separating the mixture resulting from step c) in a first phase separator vessel into a liquid phase and a helium-enriched gas phase;

step e): at least partially condensing said helium-enriched gas phase in a heat exchanger;

step f): separating the stream resulting from step e) in a second phase separator vessel into a liquid phase and a gas phase containing more than 50% by volume of helium.

According to other embodiments, the subject of the present invention is also:

• • A process as defined above, characterized in that it comprises a step g): using, as refrigerant, the liquid phase after expansion resulting from step f) in said heat exchanger used in step e).
  • A process as defined above, characterized in that, during step g), said liquid phase is vaporized at a pressure of between 0.1 bar and 3 bar absolute.
  • A process as defined above, characterized in that, during step a), the gas stream is introduced into the high-pressure column at two or more feed levels, the vapor fraction of the first feed being smaller than the vapor fraction of the second and the first feed being introduced at a higher level of said high-pressure column than the second.
  • A process as defined above, comprising a step h): extracting the gas phase resulting from step f) as a helium-rich product containing at least 75% by volume of helium.
  • A process as defined above, characterized in that the temperature at the outlet of the condenser in step b) is between −150° C. and −165° C.
  • A process as defined above, characterized in that the pressure in the high-pressure column is between 20 bar absolute and 50 bar absolute and the pressure in the low-pressure column is between 1 bar absolute and 5 bar absolute.
  • A process as defined above, characterized in that the temperature at which step c) is carried out is between −160° C. and −180° C.
  • A process as defined above, characterized in that the temperature at which step e) is carried out is between −180° C. and −210° C.
  • A process as defined above, characterized in that the liquid phase resulting from step d) is introduced into the low-pressure column of said rejection unit.
  • A process as defined above, comprising an additional step of producing a nitrogen-enriched stream comprising less than 2% by volume of methane from a gas stream from the top of the low-pressure column.
  • A process as defined above, characterized in that the source gas stream comprises from 20% by volume to 80% by volume of methane, from 20% by volume to 80% by volume of nitrogen and from 0% by volume to 2% by volume of helium.
  • A facility for producing helium from a gas source mixture comprising methane, helium and nitrogen, said facility comprising:
  • at least one nitrogen rejection unit equipped with a double column, said double column comprising a high-pressure distillation column, a low-pressure distillation column and a condenser linking the high-pressure column with the low-pressure column, and
  • a helium extraction unit comprising a first phase separator vessel located upstream of a heat exchanger which is itself located upstream of a second phase separator vessel;
  • characterized in that the nitrogen rejection unit is located upstream of the helium extraction unit.
  • A facility as previously defined, characterized in that said helium extraction unit contains neither a distillation column nor a rectification column nor a phase separator vessel with multiple feeds.

The solution which is the subject of the present invention makes it possible to separate a stream composed of methane, nitrogen and helium into three streams: a pure methane stream, a pure nitrogen stream and a helium-rich stream.

The present invention makes it possible to solve the problem of the solutions by partial condensation, by obtaining both a high helium yield (>85%) and a high helium content (>50%), this being without using a column dedicated to helium separation.

The integration of the helium extraction in an NRU unit with a double-column scheme makes it possible to minimize the cost of the equipment and the energy consumption of the helium extraction unit (helium rejection unit, HRU) without penalizing the consumption and the installation cost of the other pieces of equipment of the NRU.

The process which is the subject of the invention consists of a double-column NRU unit in which total condensation (or virtually total condensation owing to the presence of uncondensable compounds) is carried out at the level of the condenser of the high-pressure HP column. The liquid produced at the top of this HP column is partly used as reflux of the HP column and partly intended to be used as reflux of the low-pressure LP column.

The particularity of the invention is the expansion of the liquid produced and not used as reflux of the HP column, to an intermediate pressure so as to thus “flash” or instantaneously vaporize (the term “flash” gas is intended to mean: instantaneous vapor) all the desired helium in a first helium separator vessel (control of the yield) followed by a partial condensation of the gas of said “flash” (or instantaneous vapor) so as to obtain a gas having a high helium content in a second helium separator vessel.

It will advantageously be possible to produce the cold required for the partial condensation of the “flash” (instantaneous vapor) from the first helium separator vessel by vaporizing at low pressure the liquid from the bottom of the second helium separator vessel.

BRIEF DESCRIPTION OF THE DRAWING

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:

FIG. 1 is a schematic representation of one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The process which is the subject of the present invention is illustrated in FIG. 1.

In FIG. 1, a source gas stream 1 comprising at least helium, methane and nitrogen is introduced at a pressure of approximately 50 bar absolute into a heat exchanger 2. The stream 3 at the outlet of said exchanger 2 is expanded by means, for example, of a valve 4, before being introduced into a high-pressure column 5 of a double-column unit 6 comprising a high-pressure distillation column 5, a low-pressure distillation column 7 and a condenser 8 linking the high-pressure column 5 with the low-pressure column 7.

The source gas stream 1 comprises, for 100% by volume, for example, from 20% by volume to 80% by volume of methane, from 20% by volume to 80% by volume of nitrogen and from 1 ppm by volume to 1% by volume of helium.

The pressure of the high-pressure column is for example 30 bara and the pressure in the low-pressure column is for example 2 bara.

The principle of a double-column unit is widely described in the technical art, for example in the document by Ruheman, “The separation of gases” Oxford University press, 1949, chapter 7 or else in the document by Barron, “Cryogenic systems”, McGraw Hill, Inc., 1996, p. 230, Air Separation Systems.

The term “nitrogen rejection unit” relates to a device in which the nitrogen and the methane are separated by cryogenic rectification.

In order to increase the helium yield, it is also possible to carry out a multiple introduction into the high-pressure column 5: a part (3d) of the stream 3 being introduced mainly in liquid form and a part (3c) is introduced mainly in gas form at the bottom of column 5 so as to vaporize the helium that would otherwise be entrained in the liquid methane.

A part 9 of the liquid stream 11 leaving the condenser 8 located in the bottom part of the low-pressure column 7 is used as reflux in the top part 10 of the high-pressure column 5. In a conventional double-column NRU scheme, the remainder of the liquid 11 is cooled and then expanded so as to be directly used as reflux of the low-pressure column. The subject matter of the present invention consists in using a part 12 of said liquid stream 11 for extracting therefrom the helium before using it as reflux of the low-pressure column.

This liquid stream 11 typically contains less than 2% by volume of methane, more that 95% by volume of nitrogen and from 0.5% by volume to 3% by volume of helium.

The temperature of the liquid stream 12 is for example between −150° C. and −165° C.

This liquid stream 12 is expanded to an intermediate pressure by means, for example, of a valve 13. This intermediate pressure is typically about 8 bara to 15 bara. It is for example 12 bara.

By virtue of this expansion, the liquid stream has been vaporized, that is to say that gas has been instantaneously formed (this is “flash” gas). The stream 14 thus produced contains a majority of liquid and a minority of gas. This gas is enriched in helium. This helium-enriched gas comprises at least 80% by volume of the helium contained in the liquid stream 12. The two-phase stream 14 is introduced into a first phase separator vessel 15. The vessel 15 produces a liquid stream 16 and a gas stream 17. The gas stream 17 contains more than 80% by volume of the helium contained in the source stream 1.

Nevertheless, this gas stream 17 contains nitrogen. The liquid stream 16 containing a majority of nitrogen, but also helium and methane, is introduced into the top part 18 of the low-pressure column 7 after having been expanded, for example by means of a valve 19, in order to serve as reflux of the low-pressure column 7. The stream 20 at the outlet of the top 21 of the column 7 is introduced into a heat exchanger 22 or 2 in order to produce a stream 23 which is rich in nitrogen or even pure with respect to nitrogen, which does not contain helium and which contains less than 1.5% by volume of methane.

The helium-enriched gas stream 17 typically contains more than 70% by volume of nitrogen, more than 5% by volume of helium and less than 2% by volume of methane, for example 90% by volume of nitrogen, from 8% to 10% by volume of helium and less than 0.5% by volume of methane. This gas stream 17 is introduced into a heat exchanger 24 in order to be at least partially condensed. The mixture 25 thus condensed is introduced into a second phase separator vessel 26. A gas stream 27 is extracted at the top of the separator 26. This gas stream 27 comprises more than 50% by volume of helium, preferably more than 60% by volume of helium and more particularly more than 70% by volume of helium. The gas stream 27 can optionally again pass through a heat exchanger 24.

The liquid phase 28 from the separator 26 serves to cool the exchanger 24 after having been expanded to a pressure of less than 3 bara, for example by means of a pressure regulator such as a valve 29. This liquid stream 28 is pure with respect to nitrogen, for example it contains more than 99% by volume of nitrogen.

By virtue of the facility 31 comprising the nitrogen rejection unit 6 and the helium extraction unit 30 as illustrated in FIG. 1 and according to a process which is the subject of the invention as previously described, with a source stream 1 containing approximately 50% by volume of nitrogen, 50% by volume of methane and 0.3% by volume of helium, a helium purity in the stream 27 of approximately 74% by volume and a helium yield of approximately 91% are obtained.

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.

Claims

1.-14. (canceled)

15. A process for producing a helium gas stream from a source gas stream comprising at least helium, methane and nitrogen, the process comprising the following steps: step a): introducing the source gas stream into a double-column nitrogen rejection unit, the double column comprising a high-pressure distillation column, a low-pressure distillation column and a condenser linking the high-pressure column with the low-pressure column, the condenser comprising an outlet, and the high-pressure column comprising a top, and the low-pressure column comprising a top;step b): extracting at the outlet of the condenser at least one part of a mixture produced at the top of the high-pressure column;step c): expanding the mixture resulting from step b) to an intermediate pressure of between 8 bar and 20 bar absolute;step d): separating the intermediate pressure mixture resulting from step c) in a first phase separator vessel into a liquid phase and a helium-enriched gas phase;step e): at least partially condensing the helium-enriched gas phase in a heat exchanger;step f): separating the at least partially condensed stream resulting from step e) in a second phase separator vessel into a liquid phase and a gas phase containing more than 50% by volume of helium.

16. The process of claim 15, further comprising a step g): using, as refrigerant, the liquid phase after expansion resulting from step f) in the heat exchanger used in step e).

17. The process of claim 16, wherein, during step g), the liquid phase is vaporized at a pressure of between 0.1 bar and 3 bar absolute.

18. The process of claim 15, wherein, during step a), the gas stream is introduced into the high-pressure column at at least a first feed level and a second feed level, the vapor fraction at the first feed level being smaller than the vapor fraction at the second feed level and the first feed level being introduced at a higher level of the high-pressure column than the second feed level.

19. The process of claim 16, further comprising a step h): extracting the gas phase resulting from step f) as helium-rich product containing at least 75% by volume of helium.

20. The process of claim 19, wherein the temperature at the outlet of the condenser in step b) is between −150° C. and −165° C.

21. The process of claim 20, wherein the pressure in the high-pressure column is between 20 bar absolute and 50 bar absolute and the pressure in the low-pressure column is between 1 bar absolute and 5 bar absolute.

22. The process of claim 15, wherein the temperature at which step c) is carried out is between −160° C. and −180° C.

23. The process of claim 15, wherein the temperature at which step e) is carried out is between −180° C. and −210° C.

24. The process of claim 15, wherein the liquid phase resulting from step d) is introduced into the low-pressure column.

25. The process of claim 15, further comprising an additional step of producing a nitrogen-enriched stream comprising less than 2% by volume of methane from a gas stream rom the top of the low-pressure column.

26. The process of claim 15, wherein the source gas stream comprises from 20% by volume to 80% by volume of methane, from 20% by volume to 80% by volume of nitrogen and from 0% by volume to 2% by volume of helium.

27. A facility for producing helium from a gas source mixture comprising methane, helium and nitrogen, the facility comprising: at least one nitrogen rejection unit equipped with a double column, the double column comprising a high-pressure distillation column, a low-pressure distillation column and a condenser linking the high-pressure column with the low-pressure column, anda helium extraction unit comprising a first phase separator vessel located upstream of a heat exchanger which is itself located upstream of a second phase separator vessel;

28. The facility of claim 27, wherein the helium extraction unit contains neither a distillation column nor a rectification column nor a phase separator vessel with multiple feeds.