Process and apparatus for the production of liquid methane

Abstract

A process and apparatus (100) for producing nitrogen-purified liquid methane from a feed gas stream (1) containing at least 98 mol% methane and between 0.1 and 2 mol% nitrogen, wherein the feed gas stream (1) is cooled to a temperature between 110 K and 200 K in a first counter-current heat exchanger (3) with a nitrogen vapor stream (9) to produce a cooled stream (5), a liquid nitrogen bath (21) is in heat exchange with the cooled stream (5) via a second heat exchanger (7) through which the cooled stream (5) passes, the liquid nitrogen bath (21) producing nitrogen vapor (9) supplied to the first heat exchanger (3) to produce the cooled stream, the cooled stream (5) exiting the second heat exchanger (7) is introduced into a distillation column (11) for distillation, and a liquid (13) containing at least 99.5% mol, preferably at least 99.9% mol,Methane is drawn off from the distillation column (11) in the tank as the final product. (Shorthand figure: Fig. 1)

Description

Title of the invention: Process and apparatus for the production of liquid methane.

[0001] The invention relates to a process and apparatus for the production of liquid methane.

[0002] The invention relates more particularly to a process for producing nitrogen-purified liquid methane from a feed gas stream containing at least 95% mol of methane and between 0.1 and 5% mol of nitrogen, in particular at least 98% mol of methane and between 0.1 and 2% mol of nitrogen.

[0003] Biogas is the gas produced during the decomposition of organic matter in the absence of oxygen (anaerobic fermentation), also known as methanation. This can be a natural decomposition – as observed in marshes or municipal waste landfills – but biogas production can also result from the methanation of waste in a dedicated reactor, with controlled conditions, called a methanizer or digester, and then in a post-digester, similar to the digester and allowing the methanation reaction to be carried out further.

[0004] Biomass will be defined as any grouping of organic matter that can be transformed into energy through this methanization process, for example: sewage sludge, manure / slurry, agricultural residues, food waste...

[0005] Biogas mainly contains methane (CH4) and carbon dioxide (CO2) in varying proportions depending on the method of production and the substrate, but may also contain, in smaller proportions, water, nitrogen, oxygen, hydrogen sulfide (H2S) or volatile organic compounds (VOCs).

[0006] Depending on the organic matter degraded and the techniques used, the proportions of the components differ, but on average biogas contains, on a dry gas basis, 30 to 75% methane, 15 to 60% CO2, up to 15% nitrogen, up to 5% oxygen and trace compounds.

[0007] Further purification of biogas allows for its wider use; in particular, further purification of biogas makes it possible to obtain biogas purified to the specifications of natural gas, which can then be substituted for it; this purified biogas is "biomethane." Biomethane thus complements natural gas resources with a renewable component produced within local areas; it can be used for exactly the same purposes as natural gas of fossil origin. It can supply a natural gas network, a vehicle refueling station, and it can also be liquefied for storage and transport as liquefied natural gas (bioLNG)...

[0008] The "conventional" purification process (membrane or water scrubbing column) allows the removal of CO2 up to ~1 to 2.5%. An adsorption step at temperature Ambient air allows its concentration to be lowered to < 50 ppm. However, gases from the air tend to remain with the methane during the purification stages. The remaining air can be removed, for example by means of a catalyst, until only a small amount remains, possibly as low as 1 to 2%.

[0009] When very high methane purity (>99.9%, for example) and a good recovery rate (>99%) are required, these purification bricks do not allow these objectives to be met. The present invention describes a solution for liquefying biomethane and purifying it into nitrogen using the same installation. The invention can also be applied to the liquefaction of any methane source containing nitrogen, for example, natural gas or a mixture of natural gas and biomethane.

[0010] The document published under number EP 3465035 A1 describes a cryogenic distillation process as a step in purifying biogas containing relatively high amounts of air (nitrogen and oxygen) (between 3 and 50 mol% nitrogen and oxygen). The target methane concentrations at the outlet are those compatible with the specifications for reinjection into the natural gas network, or for use as vehicle fuel, which corresponds to [CH4]mol > 97.5%. In particular, it proposes, through the design of cryogenic distillation, to solve the problem of explosive atmospheres that could be created with conventional distillation when purifying biogas rich in air.

[0011] The documents published under numbers FR 2971331 A1 and FR 2971332 A1 also propose cryogenic distillation processes for the purification of biogas containing between 65 and 97% methane, the remainder being air. To prevent the formation of an explosive mixture in the column, the gaseous and / or liquid mixtures in the distillation column are diluted by reinjecting some of the liquid methane into the column vessel or by injecting nitrogen gas from an external source into the vessel.

[0012] The prior art described above proposes cryogenic distillation solutions as a biogas purification step containing relatively high quantities of air (nitrogen and oxygen) to avoid the risk of explosion inside a distillation column. However, these documents do not address the liquefaction of methane as such, particularly with the removal of residual nitrogen, or the optimization of a design for this purpose.

[0013] To this end, the method according to the invention, which otherwise conforms to the generic definition given in the preamble above, is essentially characterized in that: - the feed gas stream is cooled to a temperature between 110 K and 200 K, specifically between 130 K and 150 K, in the first counter-current heat exchanger using a nitrogen vapor stream to produce a cooled stream, - a liquid nitrogen bath is in heat exchange with the cooled stream via a second heat exchanger through which the cooled stream passes; the liquid nitrogen bath produces nitrogen vapor which is supplied to the first heat exchanger to produce the cooled stream. - the cooled stream exiting the second heat exchanger is introduced into a distillation column for distillation, and - a liquid containing at least 99.5% mol, preferably at least 99.9% mol, of methane is withdrawn from the distillation column tank as the final product.

[0014] Furthermore, embodiments of the invention may include one or more of the following features: - The feed gas stream contains at least 99% mol of methane, in particular at least 99.5% mol of methane and between 0.1 and 1% mol of nitrogen, in particular between 0.1 and 0.5% mol of nitrogen. - Before being introduced into the distillation column, the cooled stream exiting the second heat exchanger passes through a boiler in the column. - A vapor phase depleted in methane and enriched in nitrogen relative to the feed gas stream is drawn off at the top of the distillation column and sent to a condenser in which the vapor phase is cooled by heat exchange with liquid nitrogen in order to condense part of the vapor phase forming a gas enriched in nitrogen relative to the vapor phase and a liquid enriched in methane relative to the vapor phase. - The liquid enriched with methane is returned to the distillation column. - Liquid nitrogen is vaporized in the condenser forming nitrogen vapor which is supplied to the first heat exchanger to produce the cooled stream. - The condenser includes a plate heat exchanger immersed in liquid nitrogen. - The liquid drawn off from the distillation column tank is cooled in a third heat exchanger by heat exchange with liquid nitrogen, liquid nitrogen being vaporized forming nitrogen vapor which is supplied to the first heat exchanger to produce the cooled stream. - The cooled stream is introduced into the upper part of the distillation column.

[0015] The invention also relates to an apparatus for producing nitrogen-purified liquid methane from a feed gas stream containing at least 95 mol% of methane and between 0.1 and 5 mol% nitrogen, in particular at least 98 mol% methane and between 0.1 and 2 mol% nitrogen, the installation comprising: - a first heat exchanger configured to cool the feed gas stream to a temperature between 110 K and 200 K, in particular between 130 K and 150 K, counter-current to a nitrogen vapor stream to produce a cooled stream, - a second heat exchanger immersed in a liquid nitrogen bath configured to vaporize the liquid nitrogen by heat exchange with the cooled stream to produce nitrogen vapor, - a distillation column configured to distill the cooled stream and produce a liquid containing at least 99.5 mol%, preferably at least 99.9 mol%, of methane in the column vessel, - a column reboiler configured to receive the cooled stream before its introduction into the distillation column, - a condenser comprising a plate heat exchanger immersed in liquid nitrogen, the condenser being configured to cool a vapor phase drawn from the top of the distillation column by heat exchange with liquid nitrogen, to form a gas enriched in nitrogen relative to the vapor phase and a liquid enriched in methane relative to the vapor phase, and to produce nitrogen vapor, - at least one first duct configured to send the cooled flow exiting the first heat exchanger to the second heat exchanger, - at least one second duct configured to send nitrogen vapor exiting the second heat exchanger to the first heat exchanger, - at least one third duct configured to send the cooled flow exiting the second heat exchanger to the boiler, - at least one fourth conduit configured to send the cooled flow exiting the boiler to the distillation column, - at least one fifth conduit configured to draw the vapor phase from the top of the distillation column and send it to the condenser, - at least one sixth conduit configured to return the methane-enriched liquid formed in the condenser to the distillation column, - at least one seventh conduit configured to send the nitrogen vapor exiting the condenser to the first heat exchanger, and - at least one eighth conduit configured to draw off liquid containing at least 99.5 mol%, preferably at least 99.9 mol% methane in the column tank as the final product.

[0016] According to other possible features: - The apparatus includes a third heat exchanger immersed in a liquid nitrogen bath configured to cool the liquid drawn into the tank from the distillation column and vaporize the liquid nitrogen to produce nitrogen vapor, and less a ninth conduit configured to send the nitrogen vapor produced to the first heat exchanger. - The device includes a liquid nitrogen storage connected to the second heat exchanger and / or the third heat exchanger and / or the condenser.

[0017] The present invention provides a solution for liquefying a methane-rich stream, particularly one from biogas and / or natural gas purification, and containing nitrogen, particularly residual nitrogen, and for efficiently purifying it of nitrogen in a single installation. It is possible to liquefy and purify a gas stream containing at least 95 mol%, in particular at least 99.5 mol%, of methane and 5% or less of nitrogen, in particular 0.5 mol% or less of nitrogen, by cryogenic distillation. The creation of an explosive atmosphere due to oxygen is not a concern.

[0018] Other features and advantages will become apparent from the following description, made with reference to the figure.

[0019] [Fig-1] illustrates an example of an apparatus and a method according to the invention of schematically.

[0020] A gas stream 1 is fed into the apparatus 100 to produce nitrogen-purified liquid methane 13. The feed gas stream 1 contains at least 95 mol% methane and up to 5 mol% nitrogen. In particular, the feed gas stream 1 contains at least 98 mol% methane and between 0.1 and 2 mol% nitrogen. More particularly, the feed gas stream 1 contains at least 99 mol% methane and between 0.1 and 1 mol% nitrogen. Even more particularly, the feed gas stream 1 contains at least 99.5 mol% methane and between 0.1 and 0.5 mol% nitrogen.

[0021] The feed gas stream 1 is for example biomethane produced from biogas purification, for example by membrane separation, by scrubbing column for example with water or solvent, by cryogenic distillation and / or by adsorption for example at ambient temperature or at modulated temperature and / or pressure.

[0022] The supply gas stream 1 can be natural gas or a mixture of natural gas and biomethane.

[0023] The gaseous feed stream 1 is cooled in a first heat exchanger 3 against the current of a nitrogen vapor stream 9. The first heat exchanger 3 is for example of the brazed aluminium plate heat exchanger type (“Brazed Aluminium Heat Exchangers” in English or BAHX).

[0024] The cooled flow 5 at the exit of the first heat exchanger 3 is at a temperature between 110 K and 200 K, in particular between 130 K and 150 K.

[0025] The cooled stream 5 exiting the first heat exchanger 3 passes into a second heat exchanger 7, which is, for example, immersed in a liquid nitrogen bath, in which the cooled stream 5 and the liquid nitrogen bath exchange heat. The cooled stream 5 heats the liquid nitrogen bath of the second heat exchanger 7 by heat exchange. This step lowers the temperature of the cooled gas stream 5 by approximately 1 K, thus allowing some of the liquid nitrogen to vaporize and be incorporated into the first heat exchanger 3 as vapor, while also utilizing the latent heat of vaporization. The nitrogen vapor 9 produced is returned to the first heat exchanger 3 to cool the feed gas stream 1 by heat exchange.

[0026] The use of a nitrogen bath makes it possible to avoid two-phase exchangers as well as direct contact between liquid nitrogen and the gas stream 5.

[0027] The level of the bath is kept constant by adding liquid nitrogen from the source 29, in order to compensate for the loss of level by vaporization.

[0028] The cooled stream 5 exiting the second heat exchanger 7 is introduced into a distillation column 11 for distillation. The cooled stream 5 can be introduced into a zone of the distillation column 11 depending in particular on its composition and the intended operating conditions, especially in the upper part of the distillation column 11.

[0029] Preferably, before being introduced into the distillation column 11, the cooled stream 5 exiting the second heat exchanger 7 passes through a reboiler 15 of the column. The cooled stream 5 recovers cooling power to complete the cooling process, while simultaneously delivering heating power to the reboiler 15 through heat exchange within the reboiler 15. The reboiler 15 of the column may be located within the column tank 11 or outside the column 11.

[0030] As illustrated, a vapor phase 17 depleted in methane and enriched in nitrogen relative to the feed gas stream 1, 5 is drawn off at the top of the distillation column 11. This vapor phase 17 can be sent to a condenser 19, which is, for example, separate from the distillation column 11. The condenser 19 may include or be composed of a plate heat exchanger immersed in liquid nitrogen 21. The vapor phase 17 is cooled by heat exchange with the liquid nitrogen 21, preferably in a liquid nitrogen bath 21, in the condenser 19 in order to condense a portion of the vapor phase 17, forming a liquid 23 enriched in methane and depleted in nitrogen relative to the vapor phase 17 and a gas 22 enriched in nitrogen and depleted in methane relative to the vapor phase 17.

[0031] The methane-enriched liquid 23 is sent to the top of column 11 to form a reflux liquid. The nitrogen-enriched gas 22 is vented to a vent line; recovery of the residual methane and / or residual cooling power can be considered.

[0032] In the condenser 19, the liquid nitrogen 21 is vaporized 9 and reintegrated into the first heat exchanger 3 to cool the feed gas stream 1 by heat exchange.

[0033] A liquid 13 containing at least 99.5 mol%, preferably at least 99.9 mol%, or even at least 99.99 mol%, of methane is withdrawn from the tank or the bottom of the distillation column 11 as the final product. The withdrawn liquid 13 can be cooled in a third heat exchanger 25 by heat exchange with liquid nitrogen 21, in particular a liquid nitrogen bath. The third heat exchanger 25 is preferably a plate heat exchanger immersed in a liquid nitrogen bath. The liquid 13 is in particular in a saturated liquid state at a temperature of about 130 K. Its temperature is lowered to about 110 K via the third heat exchanger 25. Cooling in the third heat exchanger 25 prevents the vaporization of the liquefied methane. The liquid nitrogen 21 thus vaporized 9 can be reused in the first heat exchanger 3 to cool the feed gas stream 1 by heat exchange.

[0034] The process according to the invention, in particular the cooling of the streams and the distillation, is preferably carried out in a cold box (dotted box in the figure). The apparatus 100, in particular the heat exchangers 3, 7, 19, 25 and the distillation column 11, is preferably kept in the cold box.

[0035] The liquid nitrogen 21 used in the process according to the invention is preferably pressurized in order to avoid a temperature which would cause the methane to freeze.

[0036] The process according to the invention makes it possible to pool the refrigerant source for the condenser, (pre-)cooling and liquefaction, and to integrate the refrigerant (nitrogen) vapor flow in the same (pre-)cooling exchanger (the first heat exchanger 3).

[0037] It is possible to achieve a methane molecule recovery rate of over 99.9%, while minimizing refrigerant consumption.

Claims

Demands

1. A process for producing nitrogen-purified liquid methane from a feed gas stream (1) containing at least 95 mol% methane and between 0.1 and 5 mol% nitrogen, in particular at least 98 mol% methane and between 0.1 and 2 mol% nitrogen, wherein: - the feed gas stream (1) is cooled to a temperature between 110 K and 200 K, in particular between 130 K and 150 K, in a first counter-current heat exchanger (3) with a nitrogen vapor stream (9) to produce a cooled stream (5), - a liquid nitrogen bath (21) is in heat exchange with the cooled stream (5) via a second heat exchanger (7) through which the cooled stream (5) passes, the liquid nitrogen bath (21) producing nitrogen vapor (9) supplied to the first heat exchanger (3) to produce the stream cooled, - the cooled stream (5) exiting the second heat exchanger (7) is introduced into a distillation column (11) for distillation,and - a liquid (13) containing at least 99.5 mol%, preferably at least 99.9 mol%, of methane is withdrawn from the distillation column (11) in the tank as the final product.

2. A method according to claim 1 in which, before introduction into the distillation column (11), the cooled stream (5) exiting the second heat exchanger (7) passes through a boiler (15) of the column.

3. A process according to claim 1 or 2 wherein a vapor phase (17) depleted in methane and enriched in nitrogen relative to the feed gas stream is withdrawn from the top of the distillation column and sent to a condenser (19) in which the vapor phase (17) is cooled by heat exchange with liquid nitrogen (21) in order to condense a portion of the vapor phase (17) forming a gas (22) enriched in nitrogen relative to the vapor phase (17) and a liquid (23) enriched in methane relative to the vapor phase (17).

4. A method according to claim 3 wherein the liquid (23) enriched in methane is returned to the distillation column.

5. A method according to claim 3 or 4 wherein liquid nitrogen (21) is vaporized in the condenser (19) forming nitrogen vapor (9) which is supplied to the first heat exchanger (3) to produce the cooled flow (5).

6. A method according to any one of claims 3 to 5 wherein the condenser (19) comprises a plate heat exchanger immersed in liquid nitrogen.

7. A method according to any one of claims 1 to 6 wherein the liquid (13) drawn from the distillation column (11) into the tank is cooled in a third heat exchanger (25) by heat exchange with liquid nitrogen (21), with liquid nitrogen being vaporized forming nitrogen vapor (9) which is supplied to the first heat exchanger (3) to produce the cooled stream.

8. A method according to any one of claims 1 to 7 in which the cooled stream (5) is introduced into the upper part of the distillation column (11).

9. Apparatus (100) for producing nitrogen-purified liquid methane from a feed gas stream (1) containing at least 95 mol% methane and between 0.1 and 5 mol% nitrogen, in particular at least 98 mol% methane and between 0.1 and 2 mol% nitrogen, the installation comprising: - a first heat exchanger (3) configured to cool the feed gas stream (1) to a temperature between 110 K and 200 K, in particular between 130 K and 150 K, counter-current to a nitrogen vapor stream (9) to produce a cooled stream (5), - a second heat exchanger (7) immersed in a liquid nitrogen bath (21) configured to vaporize the liquid nitrogen (21) by heat exchange with the cooled stream (5) to produce nitrogen vapor (9), - a distillation column (11) configured to distill the cooled stream (5) and produce a liquid (13) containing at least 99.5 mol%, preferably at least 99.9 mol%, of methane in the column vessel (11),- a boiler (15) of the column configured to receive the cooled stream (5) before its introduction into the distillation column (11), - a condenser (19) comprising a plate heat exchanger immersed in liquid nitrogen (21), the condenser being configured to cool a vapor phase (17) drawn off at the top of the distillation column by heat exchange with the liquid nitrogen (21), for, to form a gas (22) enriched in nitrogen relative to the vapor phase (17) and a liquid (23) enriched in methane relative to the vapor phase (17), and to produce nitrogen vapor (9), - at least a first conduit configured to send the cooled stream (5) exiting the first heat exchanger (3) to the second heat exchanger (7), - at least a second conduit configured to send the nitrogen vapor (9) exiting the second heat exchanger (7) to the first heat exchanger (3), - at least a third conduit configured to send the cooled stream (5) exiting the second heat exchanger (7) to the boiler (15), - at least a fourth conduit configured to send the cooled stream (5) exiting the boiler (15) to the distillation column (11), - at least a fifth conduit configured to draw off the vapor phase (17) at the top of the distillation column and send it to the condenser (19),- at least one sixth conduit configured to return the methane-enriched liquid (23) formed in the condenser (19) to the distillation column (11), - at least one seventh conduit configured to send the nitrogen vapor (9) exiting the condenser (19) to the first heat exchanger (3), and - at least one eighth conduit configured to withdraw the liquid (13) containing at least 99.5 mol%, preferably at least 99.9 mol%, of methane from the column tank as the final product.

10. Apparatus (100) according to claim 9 comprising a third (25) heat exchanger immersed in a liquid nitrogen bath (21) configured to cool the liquid (13) drawn into the tank from the distillation column and vaporize the liquid nitrogen (21) to produce nitrogen vapor (9), and at least a ninth conduit configured to send the nitrogen vapor (9) produced to the first heat exchanger (3).

11. Apparatus (100) according to claim 9 or 10 comprising a liquid nitrogen storage (29) connected to the second heat exchanger (7) and / or the third heat exchanger (25) and / or the condenser (19).

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