PROCESS FOR CRYOGENIC SEPARATION OF A FEED STREAM CONTAINING METHANE AND AIR GASES, FACILITY FOR PRODUCING BIOMETHANE BY PURIFICATION OF BIOGASES DERIVED FROM NON-HAZARDOUS WASTE STORAGE FACILITIES (NHWSF) IMPLEMENTING THE PROCESS

Abstract

A process for cryogenic separation of a feed stream containing methane and air gases in which: the feed stream is cooled in order to produce a cooled stream, at least one portion of the cooled stream is sent to one level of a distillation column, a bottom stream is drawn off from the distillation column, the bottom stream being enriched in methane relative to the feed stream, a stream enriched in oxygen and in nitrogen relative to the feed stream is drawn off from the distillation column, at least one noncombustible dilution stream that is more volatile than oxygen is introduced into the distillation column at at least one level lower than the one at which the cooled stream is introduced. The dilution stream is extracted from the feed stream. Facility for producing biomethane by purification of biogases derived from non-hazardous waste storage facilities (NHWSF) implementing the process.

Claims

1-11. (canceled)

12. A method for cryogenic separation in a distillation column of a feed stream containing a mixture of methane and air gases comprising nitrogen and oxygen, the distillation column comprising a distillation head at the upstream most portion of the distillation column and an oppositely disposed distillation bottom, and one or more distillation segments arranged between the distillation head and the distillation bottom, the method comprising: flowing the feed stream through a heat exchanger to cool the feed stream thereby forming a cooled feed stream; flowing the cooled feed stream into a reboiler condenser to further cool the feed stream by heat exchange thereby condensing the feed stream into a condensed stream; decompressing the condensed stream to an operating pressure of the distillation column thereby forming a decompressed stream; flowing the decompressed stream into a separator wherein the decompressed stream is separated in a gaseous phase and a liquid phase, wherein the cooled feed stream is cooled in the reboiler condenser such that upon separation in the separator, nitrogen in the decompressed stream preferentially separates into the gaseous phase and methane preferentially separates into the liquid phase such that concentrations of nitrogen, methane, and oxygen in the gaseous phase result in a nonflammable mixture; flowing the liquid phase into the distillation head such that the liquid phase flows downstream through the one or more distillation segments for distillation; flowing the gaseous phase into the distillation column downstream at least one of the distillation segments, wherein the gaseous phase flows into the distillation column as a sweeping gas and participates in distillation by diluting a concentration of gaseous oxygen present in the distillation column to thereby maintain a mixture of nitrogen, methane and oxygen within the distillation column that is nonflammable; drawing, after distillation, a methane-rich bottom liquid stream off of the bottom of the distillation column and a methane-depleted gaseous stream off the distillation column at the distillation head, wherein the methane-rich bottom liquid stream is enriched in methane as compared to the feed stream; wherein: a portion of the methane-rich bottom liquid stream is flowed into the reboiler condenser to cool by heat exchange the cooled feed stream, and the portion of the methane-rich liquid bottom stream is vaporized in the reboiler condenser into a methane-rich gaseous phase that is flowed into the distillation column downstream of a location at which the gaseous phase from the separator is flowed into the distillation column as the sweeping gas.

13. The method of claim 12, further comprising mixing a decompressed liquid nitrogen stream with the decompressed stream before the decompressed stream is flowed into the separator.

14. The method of claim 12, further comprising flowing a refrigerant fluid into the distillation column to cool the distillation column.

15. The method of claim 14, wherein the refrigerant fluid is flowed into the distillation head.

16. The method of claim 14, wherein the refrigerant fluid is liquid nitrogen.

17. The method of claim 12, further comprising flowing the feed stream through at least one purification unit before flowing the feed stream through the heat exchanger, wherein the purification unit is loaded with an adsorbent capable of reversibly adsorbing CO.sub.2 to thereby reduce a CO.sub.2 concentration of the feed stream.

18. The method of claim 17, wherein the purification unit is a unit for purification by adsorption of PSA or PTSA and wherein the PSA or PTSA is regenerated by flowing the methane-depleted gaseous stream drawn off the distillation column through the purification unit.

19. The method of claim 12, wherein the distillation column comprises at least two distillation segments, wherein the gaseous phase is flowed into the distillation column between the adjacent ones of the at least two distillation segments and the methane-rich gaseous phase from the reboiler condenser is flowed below both of the adjacent ones of the at least two distillation segments.

20. The method of claim 12, wherein the condensed stream is decompressed to a pressure between 1 and 5 bar absolute.

21. The method of claim 12, wherein the methane-depleted gaseous stream drawn off of the distillation column at the distillation head is flowed into the heat exchange to cool the feed stream by heat exchange.

22. The method of claim 12, wherein the method is free of a flow of gaseous nitrogen into the distillation column from an external source for dilution of oxygen within the distillation column.

23. A facility for producing biomethane by purifying biogas from non-hazardous waste storage facilities (NHWSF) implementing the method according to claim 12 and comprising successively: the heat exchanger for cooling the feed stream; the reboiler condenser for condensing the cooled feed stream by heat exchange with the methane-rich bottom liquid stream drawn off the distillation column; a decondenser for decondensing the condensed stream; the separator, wherein the separator is a round-bottom separator for separating the decompressed stream into the liquid and gas phases; the distillation column; a conduit for transporting the liquid phase from the round-bottom separator into the distillation head of the distillation column; a conduit for transporting the gaseous phase from the separator into the distillation column; and a conduit for withdrawing the methane enriched stream from the bottom of the distillation column.

24. The facility of claim 23, further comprising a purification unit upstream of the heat exchanger for reducing a concentration of CO.sub.2 in the feed stream.

25. The facility of claim 23, further comprising a refrigerant fluid storage unit and a conduit for flowing the refrigerant fluid into the distillation column and/or into a conduit for mixing with the decondensed stream prior to introduction of the decondensed stream into the separator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The disclosed process and facility and resulting benefits will become clear from the following examples supported by the attached figures.

[0073] FIG. 1 shows the explosiveness diagram of a methane, oxygen and nitrogen mixture.

[0074] FIGS. 2 to 4 are diagrams of a cryogenic separation unit integrated into a facility (partially shown) for production of biomethane by purification of biogas coming from nonhazardous waste storage (NHWS). The drawings differ in the final use of the streams after cryogenic separation.

DETAILED DESCRIPTION

[0075] The explosiveness diagram for a methane, oxygen and nitrogen mixture is shown in FIG. 1. In this diagram, the explosiveness zone is grayed. The composition of the gas phase over the entire height of the column is shown with a dashed line for the case where the round bottom separator is not installed; the vapor phase then crosses into the explosiveness zone. In the scenario where a round bottom phase separator is installed, and where the gas phase produced is used to sweep the packing, then the gas phase rising in the column is not explosive.

[0076] For each of FIGS. 2 to 4, a feed gas stream (1), at a pressure included between 5 and 15 bar absolute, comprising between 60 and 97% methane, between 3 and 50% nitrogen and oxygen, and 3% or less carbon dioxide is added into the unit for purification by adsorption (2), advantageously a PTSA, in order to lower the water and carbon dioxide concentration to a value less than or equal to 50 ppmv.

[0077] The stream thus produced (3) is then cooled in a heat exchanger (4) by exchanging heat with the methane enriched liquid stream (20) and the methane depleted stream (23). The cooled stream (5) is sent into a reboiler condenser (6) where it is cooled again by heat exchange with the bottom liquid, so that the bottom liquid can boil and generate methane-rich gas which will be used in the distillation, and also to condense the feed stream.

[0078] The condensed stream (7) is then decompressed in a decompression member (8) to the operating pressure of the column (18), included between 1 and 5 bar absolute.

[0079] According to FIGS. 2 and 3, a liquid nitrogen stream (13) coming from liquid nitrogen storage (12) is decompressed in a decompression member (14) and the decompressed stream (15) is mixed with the feed stream (9), at point 16, to next be introduced in a round bottom container for separation of liquid and gas phases (11). In an embodiment not shown, the liquid nitrogen is injected directly in the round-bottom separator.

[0080] The embodiment differs from the one shown in FIG. 4 in that the liquid nitrogen is injected directly in the upper part of the distillation column, by means, for example, of an injection nozzle (16).

[0081] For all embodiments, the liquid nitrogen rich liquid phase (19) coming from the round-bottom separator (11) is next added into the upper part of the distillation column (18). The gas phase (17) is added into the lower part of the packing of the distillation column (18) to constitute the sweeping gas and participate in the distillation.

[0082] Distillation thus produces two streams: a methane enriched stream (20), bottom of the distillation column, and a methane depleted stream (23) that is rich in O.sub.2 and N.sub.2 at the head of the distillation column.

[0083] A fraction of the methane enriched liquid stream (20) is sent to the exchanger (4) to be vaporized and form a gaseous stream (22). This gaseous stream can be used in two ways.

[0084] As shown in FIG. 2, this gaseous stream (22) is used as is. It is sent for injection into the natural gas network via an injection station, or to a compression station to produce compressed natural gas, for use in a vehicle natural gas, for example.

[0085] As shown in FIG. 3, this gaseous stream (22) is used to regenerate the purification unit (2) and form a methane enriched stream (26) containing carbon dioxide coming from regeneration of the purification unit. The stream (26) is next sent to a compression station to produce compressed natural gas, for use in a natural gas vehicle, for example.

[0086] In an embodiment not shown, the methane enriched stream (20) is drawn off in a liquid form and used as liquefied natural gas.

[0087] For all embodiments, the other fraction of the methane enriched liquid stream in column bottom is sent to the reboiler condenser (6) to be vaporized. The gas stream thus created (21) is sent to the distillation column to create the rising vapor participating in the distillation.

[0088] The gas stream (23) comprising oxygen, nitrogen and a methane fraction is then sent to the exchanger (4) to be reheated.

[0089] In the embodiment shown in FIG. 2, the stream (24) leaving the exchanger is used to regenerate the purification unit (2) and produce the stream (25), which is then treated for burning the residual methane in an oxidizer.

[0090] If the methane concentration is over 25%, the stream (25) can be used in a cogeneration engine or micro-turbine in order to produce electricity.

[0091] In the embodiment shown in FIG. 3, the stream (25) is sent directly to the methane oxidation or usage systems mentioned above.