FR3038973B1 - HELIUM PRODUCTION FROM NATURAL GAS CURRENT - Google Patents

Abstract

Process for producing a gaseous stream (27) of helium from a source gas stream (1) comprising at least helium, methane, and nitrogen, comprising at least the following steps: Step a): introducing said source gas stream (1) into a double column nitrogen discharge unit (6), said double column comprising a high pressure distillation column (5), a low distillation column (7) pressure and a condenser (8) connecting the column (5) at high pressure with the column (7) at low pressure; Step b): extraction at the outlet of said condenser (8) of at least a portion (12) of a mixture (11) produced at the top of column (5) at high pressure; Step c): expanding said mixture from step b) to an intermediate pressure of between 8 bar and 20 bar absolute; Step d): separating the mixture (14) from step c) in a first phase separator pot (15) into a liquid phase (16) and a gas phase (17) enriched in helium; Step e): at least partial condensation of said gas phase (17) enriched in helium in a heat exchanger (24); Step f): separation of the stream (25) from step e) into a second phase separator pot (26) into a liquid phase (28) and a gas phase (27) containing more than 50% by volume of helium.

Description

The present invention relates to a method for producing helium from a source gas stream comprising at least helium, methane and nitrogen. Helium is obtained commercially almost exclusively from a mixture of volatile components of natural gas, this mixture comprising, as well as helium, typically methane and nitrogen and traces of hydrogen, argon and other noble gases.

During the production of mineral oil, helium is made available as a component of the gas that accompanies mineral oil, or as part 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 the air of the order of 5.2 ppmv).

Raw natural gas can contain a large number of troublesome impurities to be removed. Nitrogen is one example. From a certain concentration of nitrogen in natural gas, it is typically not salable because of its low calorific value. To remove the nitrogen is most often used a cryogenic process implemented in a unit called Nitrogen Rejection Unit (NRU).

If in addition to nitrogen, there is helium in natural gas, there is economic interest in extracting a helium-rich stream separately to produce helium and market it. Typically, the helium is concentrated in the cryogenic process and finally purified in a PSA (Pressure Swing Adsorption) type adsorption process.

Several solutions exist today for the extraction of helium in a NRU, in particular in a so-called double-column NRU unit.

It is possible to add a column to distill nitrogen and helium, as described in patent EP0633437 B1 for example, but this poses several problems: • Complexification of the installation. • Increased energy consumption due to the refrigeration requirement (cycle) associated with this column. Other solutions exist using partial condensations and / or flashes (flashes) successive but they generally do not allow to have both a high helium content and a high yield. Such an example is described in US4701201. In this patent, helium is recovered at the top of the high pressure column (HP) after partial condensation. This simple partial condensation makes it necessary to choose between yield and purity in helium. In the patent it is also described that it is possible to achieve a reasonable yield (about 92%) but the purity achieved for the helium-enriched stream is 37.2%.

There is therefore a need to solve the problems described above. Therefore, the present invention relates to a method 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): introduction of said source gas stream into a double column nitrogen discharge unit, said double column comprising a high pressure distillation column, a low pressure distillation column and a condenser connecting the high column pressure with the low pressure column;

Step b): extraction at the outlet of said condenser of at least a portion of a mixture produced at the top of the column at high pressure;

Step c): relaxation of said mixture resulting from step b) at an intermediate pressure of between 8 bar and 20 bar absolute

Step d): separation of the mixture from step c) in a first phase separator pot in a liquid phase and a helium-enriched gas phase;

Step e): at least partial condensation of said gas phase enriched in helium in a heat exchanger;

Step f): separation of the stream from step e) in a second phase separator pot in a liquid phase and a gas phase containing more than 50% by volume of helium.

According to other embodiments, the present invention also relates to:

A process as defined above characterized in that it comprises a step g): use, as a refrigerant, of the liquid phase after expansion resulting from step f) in said heat exchanger implemented at step e).

A process as defined above characterized in that during step g), said liquid phase is vaporized at a pressure between 0.1 bar and 3 bar absolute.

A process as defined above characterized in that during step a) the gaseous stream is introduced into the high-pressure column at at least two feed levels, the steam fraction of the first feed being more weak that 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): extraction of the gaseous phase from step f) as a product rich in helium, containing at least 75% by volume of helium.

A process as defined above characterized in that the temperature at the condenser outlet 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 of implementation of step c) is between - 160 ° C and -180 ° C.

A process as defined above characterized in that the operating temperature of step e) is between - 180 ° C and -210 ° C.

A process as defined above characterized in that the liquid phase from step d) is introduced into the low pressure column of said reject unit.

A process as defined above comprising a further step of producing a nitrogen enriched stream comprising less than 2% by volume of methane from a gaseous stream from the low pressure column head.

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. % by volume of helium.

A plant for producing helium from a mixture of source gas comprising methane, helium, and nitrogen, said installation comprising: at least one nitrogen rejection unit provided with a double column, said double column comprising a high pressure distillation column, a low pressure distillation column and a condenser connecting the high pressure column with the low pressure column, and a helium extraction unit comprising a first pot phase separator located upstream of a heat exchanger located upstream of a second phase separator pot; characterized in that the nitrogen rejection unit is located upstream of the helium extraction unit;

An installation as defined above, characterized in that said helium extraction unit does not contain a distillation column, a rectification column or a multi-feed phase separator pot.

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

The present invention solves the problem of solutions by partial condensation by obtaining both a high yield (> 85%) and a high helium content (> 50%) and this without using a column dedicated to the separation of the helium. The integration of helium extraction in a NRU unit with a double column scheme minimizes the equipment cost and the energy consumption of the helium rejection unit (HRU) without penalizing the consumption and installation cost of other NRU equipment.

The method which is the subject of the invention consists of a double-column NRU unit in which the condenser of the HP high-pressure column is completely (or almost completely because of the presence of incondensables) condensed. The liquid produced at the top of this HP column is partly used as reflux of the HP column and partly for use as reflux of the LP low pressure column.

The particularity of the invention is to relax the fluid produced and not used as reflux of the HP column at an intermediate pressure so as to "flash" or vaporize instantly (by "flash" gas is meant: instantaneous steam) all the helium desired in a first helium separator pot (performance control) and then to a partial condensation of the gas of said "flash" (or instantaneous vapor) to obtain a gas having a high helium content in a second separator pot of helium.

It will advantageously be possible to produce the cold necessary for the partial condensation of the "flash" (instantaneous vapor) coming from the first helium separator pot by vaporizing at low pressure the tank liquid of the second helium separating pot.

The method that is the subject of the present invention is illustrated in FIG.

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 in a heat exchanger 2. The output stream 3 of said The exchanger 2 is expanded using, for example, 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 column of distillation 7 at low pressure and a condenser 8 connecting the column 5 at high pressure with the column 7 at low pressure.

The source gas stream 1 comprises 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% by volume to 1% by volume. helium volume.

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 state of the art, for example in Ruheman's paper, "The Separation of Gases" Oxford University Press, 1949, Chapter 7 or in the Barron paper, Cryogenics Systems, mcGraw Hill, Inc., 1996, p. 230, Air Separation Systems.

The term nitrogen rejection unit refers to a device in which nitrogen and methane are separated by cryogenic rectification.

To increase the helium yield, it is also possible to make 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 gaseous form in the tank column 5 to vaporize the helium that would otherwise be entrained in the liquid methane.

A portion 9 of the liquid stream 11 leaving the condenser 8 located in the lower part of the column 7 at low pressure is used as reflux in the upper part of the column 5 at high pressure. In a conventional double column NRU scheme, the remainder of the liquid 11 is cooled and then expanded to be used directly as the reflux of the low pressure column. The object of the present invention is to use a portion 12 of said liquid stream 11 to extract 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 than 95% by volume of nitrogen and 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 using, for example, a valve 13. This intermediate pressure is typically of the order of 8 bara to 15 bara. For example 12 bara.

By this expansion, the liquid stream was vaporized, that is to say that gas was formed instantly (it is "flash" gas). The stream 14 thus produced contains a majority of liquid and a minority of gases. 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 current 14 is introduced into a first phase separator pot. The pot 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 upper part 18 of the column 7 at low pressure after being expanded, for example by means of a valve 19 to serve as reflux of the column 7 at low pressure. The stream 20 at the outlet of the head 21 of the column 7 is introduced into a heat exchanger 22 or 2 in order to produce a stream 23 rich in nitrogen or even pure nitrogen containing no helium and containing less than 1.5% in volume of methane.

The gas stream 17 enriched with helium 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, 8 % to 10% by volume of helium and less than 0.5% by volume of methane. This gaseous stream 17 is introduced into a heat exchanger 24 in order to be condensed at least partially. The mixture thus condensed is introduced into a second phase separator pot 26. At the top of the separator 26, a gas stream 27 is extracted. This gaseous 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 may, optionally, be ironed in a heat exchanger 24.

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

With the installation 31 comprising the nitrogen rejection unit 6 and the helium extraction unit 30 as illustrated in FIG. 1 and according to a method which is the subject of the invention as described above, with a 1 source stream containing about 50% by volume of nitrogen, 50% by volume of methane and 0.3% by volume of helium, we obtain a purity of helium in the stream 27 of about 74% by volume and a helium yield of about 91%.

Claims (12)

claims A method for producing a gaseous stream (27) of helium from a source gas stream (1) comprising at least helium, methane, and nitrogen, comprising at least the following steps Step a): introducing said source gas stream (1) into a double column nitrogen discharge unit (6), said double column comprising a high pressure distillation column (5), a distillation column (7) at low pressure and a condenser (8) connecting the column (5) at high pressure with the column (7) at low pressure; Step b): extraction at the outlet of said condenser (8) of at least a portion (12) of a mixture (11) produced at the top of column (5) at high pressure; Step c): expanding said mixture from step b) to an intermediate pressure of between 8 bar and 20 bar absolute; Step d): separating the mixture (14) from step c) in a first phase separator pot (15) into a liquid phase (16) and a gas phase (17) enriched in helium; Step e): at least partial condensation of said gas phase (17) enriched in helium in a heat exchanger (24); Step f): separation of the stream (25) from step e) into a second phase separator pot (26) into a liquid phase (28) and a gas phase (27) containing more than 50% by volume of helium. 2. Method according to the preceding claim characterized in that it comprises a step g): use, as a refrigerant, of the liquid phase (28) after expansion from step f) in said heat exchanger (24) set work in step e). 3. Method according to claim 2 characterized in that during step g), said liquid phase (28) is vaporized at a pressure between 0.1 bar and 3 bar absolute. 4. Method according to one of the preceding claims characterized in that during step a) the gas stream is introduced into the column (5) at high pressure at least two levels of feed, the steam fraction of the first supply (3d) being lower than the vapor fraction of the second (3c) and the first supply (3d) being introduced at a higher level of said high pressure column (5) than the second (3c). 5. Method according to one of the preceding claims comprising a step h): extraction of the gas phase (27) from step f) as a product rich in helium, containing at least 75% by volume of helium. 6. Method according to the preceding claim characterized in that the condenser outlet temperature (8) in step b) is between - 150 ° C and - 165 ° C. 7. Method according to the preceding claim characterized in that the pressure in the column (5) at high pressure is between 22 bar absolute and 50 bar absolute and the pressure in the low pressure column is between 1 bar absolute and 5 bar absolute. 8. Method according to any one of the preceding claims characterized in that the operating temperature of step c) is between - 160 ° C and - 180 ° C. 9. Method according to any one of the preceding claims characterized in that the operating temperature of step e) is between - 180 ° C and-210 ° C. 10. Method according to any one of the preceding claims characterized in that the liquid phase (16) from step d) is introduced into the column (7) at low pressure of said unit (6) of rejection. The method of any of the preceding claims comprising a further step of producing a nitrogen enriched stream (23) comprising less than 2% by volume of methane from a gas stream (20) from the head. (21) of the column (7) at low pressure. 12. Process according to any one of the preceding claims, characterized in that the source gas stream (1) comprises from 20% by volume to 80% by volume of methane, from 20% by volume to 80% by volume of nitrogen and 0% by volume to 2% by volume of helium.