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. A cryogenic separation process for a feed stream containing methane and air gases comprising nitrogen and oxygen wherein: the feed stream is cooled to produce a cooled stream comprising methane, nitrogen, and oxygen, wherein the feed stream is at a pressure of 5 to 15 bar absolute when cooled; the cooled stream is at least partially condensed; the at least partially condensed cooled stream is decompressed to produce a decompressed stream, said decompressed stream producing a gaseous phase and a liquid phase and said decompressed stream having a pressure of 1 to 5 bar absolute; the gaseous phase and the liquid phase are separated; the liquid phase is injected into a level of a distillation column; a bottom stream is drawn off from the distillation column, where the bottom stream is enriched in methane compared to the feed stream; a stream, which is enriched in oxygen and nitrogen compared to the feed stream is drawn off from the distillation column, wherein: the gaseous phase, which consists essentially of N.sub.2 and which is incombustible and more volatile than oxygen, is added to the distillation column at least one level below the level at which the liquid phase is added, and wherein: the cooled stream is at least partially condensed by heat exchange with a fraction of the bottom stream which further cools the cooled stream resulting in the cooled stream being at least partially condensed and the fraction of the bottom stream being boiled to produce a methane-rich gas which is then fed into the distillation column, the gaseous phase that consists essentially of N.sub.2 is added to the distillation column above where the methane rich gas is fed into the distillation column, and the decompressed stream is mixed with a refrigerant fluid before the gaseous phase and the liquid phase are separated.

2. The process according to claim 1, wherein the distillation column contains several distillation segments and the gaseous phase is added between two segments.

3. The process according to claim 1, wherein a CO.sub.2 concentration of the feed stream is reduced by adsorption using at least one purification unit loaded with adsorbent able to reversibly adsorb CO.sub.2 before the feed stream is cooled.

4. The process according to claim 3, wherein the purification unit is a unit for purification by adsorption of PSA or PTSA and wherein the PSA or PTSA is regenerated by means of the oxygen and nitrogen-enriched gas stream drawn from a column head of the distillation column.

5. The method of claim 1, wherein the distillation column comprises two or more distillation segments, and the gaseous phase that consists essentially of N.sub.2 is added to the distillation column such that at least one of the two or more distillation segments is interposed between the gaseous phase and the methane rich gas.

6. The method of claim 5, wherein the gaseous phase that consists essentially of N.sub.2 is added between at least two of the two or more distillation segments.

7. The method of claim 5, wherein the liquid phase is introduced above of each of the two or more distillation segments, the gaseous phase consisting essentially of N.sub.2 is added between distillation segments of the two or more distillation segments, and the methane-rich gas is added below the two or more distillation segments.

8. The method of claim 1, wherein the gaseous phase that consists essentially of N.sub.2 is added such that a gaseous mixture resulting within the distillation column has an oxygen concentration below about 12%.

9. A cryogenic separation process for a feed stream containing methane and air gases comprising nitrogen and oxygen, comprising: cooling the feed stream to produce a cooled stream comprising methane, nitrogen, and oxygen; at least partially condensing the cooled stream by heat exchange; decompressing the at least partially condensed cooled stream to produce a decompressed stream comprising a gaseous phase and a liquid phase, wherein the gaseous phase consists essentially of N.sub.2; separating the gaseous phase from the liquid phase; injecting the liquid phase into a distillation column at a first level; adding the gaseous phase into the distillation column at a second level; drawing a bottom stream off of the distillation column, where the bottom stream is enriched in methane compared to the feed stream; and drawing off a stream which is enriched in oxygen and nitrogen compared to the feed stream off of the distillation column, wherein: the distillation column comprises two or more distillation segments, the gaseous phase that consists essentially of N.sub.2 is incombustible and more volatile than oxygen, and is added into the distillation column, the cooled stream is at least partially condensed by heat exchange with a fraction of the bottom stream which further cools the cooled stream resulting in the cooled stream being at least partially condensed and the fraction of the bottom stream being boiled to produce a methane-rich gas which is then added into the distillation column at a third level, the second level, at which the gaseous phase is added to the distillation column, is below the first level, at which the liquid phase is injected into the distillation column, and above the third level, at which the methane rich gas is added into the distillation column, at least one of the two or more distillation segments is interposed between the second and third levels, and the decompressed stream is mixed with a refrigerant fluid before the gaseous phase and the liquid phase are separated.

10. The method of claim 9, wherein the second level is positioned between adjacent ones of the two or more distillation segments.

11. The method of claim 10, wherein the third level is positioned below of the two or more distillation segments.

12. The method of claim 11, wherein the first level is positioned above of the two or more distillation segments.

13. The method of claim 9, wherein the gaseous phase that consists essentially of N.sub.2 is added such that a gaseous mixture resulting within the distillation column has an oxygen concentration below about 12%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosed process and facility and resulting benefits will become clear from the following examples supported by the attached figures.

(2) FIG. 1 shows the explosiveness diagram of a methane, oxygen and nitrogen mixture.

(3) 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

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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.

(9) 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).

(10) 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.

(11) 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.

(12) 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.

(13) 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.

(14) 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.

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

(16) 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.

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

(18) 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.

(19) 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.

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