Method and installation for helium production

Abstract

The invention concerns a method which consists in cooling the feed mixture, and introducing it in the vessel of a distillation column (3), partly condensing (in 4) the column head gas, cooling (in 5) the resulting gas phase so as to condense it partly, and recuperating as impure helium the gas phase resulting from that second condensation. Additionally at least a liquid is produced through a closed refrigerating cycle (8) with variable flow-rate. The invention is useful for producing helium and liquid nitrogen from a natural gas liquefying residue gas.

Description

[0001] The present invention relates firstly to a process for producing impure helium from a feed gas mixture containing helium and other heavier components, of the type in which:

[0002] the feed mixture is brought to a distillation pressure;

[0003] the feed mixture is cooled at this distillation pressure and introduced into the bottom of a distillation column;

[0004] the overhead gas in the column is partly condensed;

[0005] the resulting gas phase is cooled so as to partially condense it; and

[0006] the gas phase resulting from this second partial condensation is recovered as impure helium product.

[0007] The invention applies for example to the production of helium from a waste gas coming from a natural gas liquefier, this waste gas containing, besides helium, methane and nitrogen.

[0008] The pressures involved here are absolute pressures.

[0009] A process of this type is disclosed in U.S. Pat. No. 4,701,200, in which the impure helium is then purified by selective pressure swing adsorption (PSA).

[0010] When the helium has to be purified cryogenically and/or liquefied, there is a need for a cryogenic liquid. The object of the invention is to meet this need in a particularly effective manner.

[0011] For this purpose, the subject of the invention is a process of the aforementioned type, characterized in that at least one of said components is furthermore produced in the form of a liquid product by implementing a variable-rate closed refrigeration cycle, the rate of which depends on the output of said liquid product.

[0012] This process may comprise one or more of the following features:

[0013] the refrigeration cycle is a nitrogen cycle;

[0014] the liquid phase obtained by the second partial condensation is enriched with one of said components and the resulting fluid is recovered as second product;

[0015] said second product is at least partly liquid;

[0016] the partial condensation of the overhead gas in the column is carried out by heat exchange with pre-expanded liquid from the bottom of the column;

[0017] a fraction of the gas resulting from the vaporization of said expanded bottom liquid is added to the feed mixture; and

[0018] the distillation pressure is substantially equal to the impure helium production pressure.

[0019] The subject of the invention is also a unit for implementing the process described above. This unit, of the type comprising:

[0020] means for bringing the feed mixture to a distillation pressure;

[0021] a heat exchange line for cooling the feed mixture at this distillation pressure;

[0022] a distillation column, the bottom of which is connected to the cold end of the heat exchange line and is fitted with a top condenser;

[0023] a heat exchanger suitable for cooling and partly condensing the vapor phase coming from the condensation passages of the top condenser;

[0024] a phase separator connected to the cold end of the heat exchanger; and

[0025] means for recovering the vapor phase coming from the separator as impure helium product is characterized in that it furthermore includes a variable-rate closed refrigeration cycle for producing a variable output of a liquid product.

[0026] Further subjects of the invention are:

[0027] firstly, a pure helium production process, characterized in that an impure helium production process as defined above is carried out and the impure helium and said liquid product are sent to a unit for purifying, and optionally liquefying, helium having a cryogenic part; and

[0028] secondly, a pure helium production installation, characterized in that it comprises an impure helium production unit as defined above and a unit for purifying, and optionally liquefying, helium containing a cryogenic part, this unit being fed with the impure helium produced by the unit and with said liquid product.

[0029] An example of how the invention is implemented will now be described in conjunction with the appended drawing, the single figure of which shows schematically a helium production installation according to the invention.

[0030] The installation shown in the drawing comprises an impure helium production unit A and a unit B for purifying the impure helium. This unit B has a cryogenic part, which may be a cryogenic helium purification apparatus and/or a pure helium liquefaction apparatus.

[0031] The unit A essentially comprises a two-stage feed compressor 1, a heat exchange line 2, a distillation column 3 provided with a top condenser 4, a heat exchanger 5, two phase separators 6 and 7 and a closed nitrogen refrigeration cycle 8.

[0032] The unit A produces impure helium at 25 bar (line 9), gaseous nitrogen at 7 bar (line 10), liquid nitrogen at 8 bar (line 11), a first methane-rich waste W1 (line 12), a second methane-rich waste W2 (line 13) and a nitrogen-rich waste W3 (line 14). All the cold parts of this unit A are contained in the one cold box 15, which is thermally insulated suitably.

[0033] In operation, a low-pressure (3.9 bar) feed mixture, sent via a line 16, is compressed to 8 bar by the first stage of the compressor 1 and then compressed to 26 bar that is to say slightly above the impure helium production pressure, at the same time as a high-pressure feed mixture sent via a line 17.

[0034] The compressed mixture is cooled and partly condensed in the exchange line 2 and introduced into the bottom of the column 3. The liquid from the bottom of this column is expanded in an expansion valve 18 to approximately 4 bar and introduced into the shell 19 of the condenser 4, in order to refrigerate the latter.

[0035] This refrigeration in the condenser 4 causes the overhead gas in the column to partly condense. The uncondensed fraction is an intermediate gas containing approximately 25% helium and 75% nitrogen, with traces of methane.

[0036] This intermediate gas is cooled and partly condensed in the exchanger 5 and then separated into two phases in the separator 6.

[0037] The vapor coming from this separator contains approximately 70% helium. After being warmed in 5 and in 2, this vapor constitutes the impure helium produced by the unit A, recovered, after being warmed in 5 and then in 2, via the line 9. This gas will then be treated in the unit B for purifying, and optionally liquefying, helium.

[0038] The methane-rich gas that results from the vaporization in the shell 19, which still contains helium, is warmed in 2. The majority of this gas is recycled via a line 20 to the intake of the first stage of the compressor 1, while the remainder is discharged via the line 12 as methane-rich waste W1.

[0039] A stream of the liquid contained in the shell 19 is withdrawn via a pump 21, vaporized and warmed in the exchange line 2, and then constitutes the other methane-rich waste W2, which is discharged via the line 13.

[0040] Of course, the two methane-rich streams may be utilized, especially for their calorific value.

[0041] The liquid phase from the separator 6 contains approximately 99% nitrogen. After expansion to 8 bar in an expansion valve 22, this liquid is introduced into the separator 7. The liquid phase constitutes the liquid nitrogen production by the unit A, which is conveyed via the line 11, whereas the gas phase constitutes a nitrogen-rich waste W3, containing approximately 95% nitrogen, which is discharged via the line 14.

[0042] As shown, one portion of the liquid nitrogen is vaporized in the exchanger 5 in order to refrigerate the latter, and then, after being warmed in 2, it forms the nitrogen gas production stream recovered via the line 10.

[0043] The nitrogen refrigeration cycle 8, is a closed cycle with a variable rate independent of the rest of the installation. It comprises a cycle compressor 23 and the combination of a turbine 24 and a supercharger 25 that are mutually coupled.

[0044] The cycle nitrogen, compressed in 23 to 15 bar and supercharged in 25 to 23 bar, is cooled in the exchange line 2 down to an intermediate temperature level and expanded in 24 to 1.6 bar. The nitrogen thus cooled serves to refrigerate the exchanger 5, then continues to be warmed up in 2 and returns to the intake of the compressor 23.

[0045] The refrigeration cycle is not necessary when liquid nitrogen is produced, and the stream of cycle nitrogen is regulated according to the output of liquid nitrogen produced. This liquid nitrogen is at least partly sent via the line 11 to the purification/liquefaction unit B and, optionally, partly withdrawn from the installation via a production line 26 tapped off the line 11.

[0046] The unit B, fed with impure helium via the line 9 and with liquid nitrogen via the line 11, produces, via a line 27, pure, possibly liquid helium and, via a line 28, a waste gas W4.

[0047] Thus, the liquid nitrogen production is integrated into the unit A and especially into its cold box 15.

Claims

1-14 (canceled).

15. A process for producing helium from a feed gas mixture containing helium and other heavier components, comprising: a) pressuring said feed mixture to a distillation pressure; b) cooling said pressurized feed mixture; c) introducing said feed mixture into the bottom of a distillation column; d) partly condensing the overhead gas in the column; e) cooling the resulting gas phase so as to partially condense it; and f) recovering the gas phase resulting from this second partial condensation as a helium product, wherein at least one of said components from said feed gas mixture is further processed to a liquid product by implementing a variable-rate closed refrigeration cycle, in which the rate of said refrigeration cycle depends on the output of said liquid product.

16. The process of claim 15, wherein said refrigeration cycle is a nitrogen cycle.

17. The process of claim 15, wherein the liquid phase obtained by the second partial condensation is enriched with one of said components and in that the resulting fluid is recovered as a second product.

18. The process of claim 17, wherein said second product is at least partly liquid.

19. The process of claim 15, wherein the partial condensation of said overhead gas in the column is carried out by heat exchange with pre-expanded liquid from the bottom of the column.

20. The process of claim 19, wherein a fraction of the gas resulting from the vaporization of said expanded bottom liquid is added to the feed mixture.

21. The process of claim 20, wherein the distillation pressure is substantially equal to the helium production pressure.

22. A process for producing helium, further comprising: g) sending said helium product and said liquid product to a second unit for purifying, and optionally liquefying, helium having a cryogenic part.

23. A unit for producing helium from a feed gas mixture containing helium and other heavier components, comprising: a) means for bringing the feed mixture to a distillation pressure; b) a heat exchange line for cooling the feed mixture at this distillation pressure; c) a distillation column, the bottom of which is connected to the cold end of the heat exchange line and is fitted with a top condenser; d) a heat exchanger suitable for cooling and partly condensing the vapor phase coming from the condensation passages of the top condenser; e) a phase separator connected to the cold end of the heat exchanger; and f) means for recovering the vapor phase coming from the separator as a helium product, wherein said unit further comprises a variable-rate closed refrigeration cycle for producing a variable output of a liquid product.

24. The unit of claim 23, wherein said refrigeration cycle is a nitrogen cycle.

25. The unit of claim 23, further comprising means for enriching the liquid phase coming from the phase separator with one of said components and means for recovering the resulting fluid as a second product.

26. The unit of claim 23, wherein the column further comprises means for expanding its bottom liquid and for sending the expanded liquid into vaporization passages in the top condenser.

27. The unit of claim 26, further comprising a line for recycling the gas contained in the shell of the top condenser into said feed gas mixture.

28. An installation for producing pure helium comprising: a) a first unit for producing helium comprising: 1) means for bringing the feed mixture to a distillation pressure; 2) a heat exchange line for cooling the feed mixture at this distillation pressure; 3) a distillation column, the bottom of which is connected to the cold end of the heat exchange line and is fitted with a top condenser; 4) a heat exchanger suitable for cooling and partly condensing the vapor phase coming from the condensation passages of the top condenser; 5) a phase separator connected to the cold end of the heat exchanger; and 6) means for recovering the vapor phase coming from the separator as a helium product, wherein said unit further comprises a variable-rate closed refrigeration cycle for producing a variable output of a liquid product; and b) a second unit for purifying, and optionally liquefying, helium having a cryogenic part, wherein said second unit is fed with the helium produced by the first unit and with said liquid product.