PROCESS AND DEVICE FOR PRODUCING LIQUID METHANE

Abstract

Process and device for producing nitrogen-purified liquid methane from a feed gas stream containing at least 98 mol % of methane and between 0.1 and 2 mol % of nitrogen, in which the feed gas stream is cooled to a temperature of between 110 K and 200 K in a first heat exchanger in countercurrent with a stream of nitrogen vapour to produce a cooled stream, a bath of liquid nitrogen is in heat exchange with the cooled stream via a second heat exchanger through which the cooled stream passes, the liquid nitrogen bath producing nitrogen vapour supplied to the first heat exchanger to produce the cooled stream, the cooled stream leaving the second heat exchanger is introduced into a distillation column with a view to its distillation, and a liquid containing at least 99.5 mol % of methane is withdrawn from the bottom of the distillation column as the final product.

Claims

1. A process for the production of 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, wherein: the feed gas stream is cooled to a temperature of between 110 K and 200 K in a first heat exchanger in counter-current to a stream of nitrogen vapour 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 producing nitrogen vapour supplied to the first heat exchanger to produce the cooled stream, the cooled stream leaving the second heat exchanger is introduced into a distillation column for distillation, and a liquid containing at least 99.5 mol % of methane is withdrawn from the bottom of the distillation column as the final product.

2. The process according to claim 1, wherein, before being introduced into the distillation column, the cooled stream leaving the second heat exchanger passes through a boiler of the column.

3. The process according to claim 1, wherein a vapour phase depleted in methane and enriched in nitrogen relative to the feed gas stream is drawn off at the head of the distillation column and sent to a condenser in which the vapour phase is cooled by heat exchange with liquid nitrogen in order to condense part of the vapour phase forming a gas enriched in nitrogen relative to the vapour phase and a liquid enriched in methane relative to the vapour phase.

4. The process according to claim 3, wherein the methane-enriched liquid is returned to the distillation column.

5. The process according to claim 3, wherein the liquid nitrogen is vaporised in the condenser forming nitrogen vapour which is supplied to the first heat exchanger to produce the cooled stream.

6. The process according to claim 3, wherein the condenser comprises a plate heat exchanger immersed in liquid nitrogen.

7. The process according to claim 1, wherein the liquid withdrawn from the bottom of the distillation column is cooled in a third heat exchanger by heat exchange with liquid nitrogen, liquid nitrogen being vaporised to form nitrogen vapour which is supplied to the first heat exchanger to produce the cooled stream.

8. The process according to claim 1, wherein the cooled stream is introduced into the upper part of the distillation column.

9. A device 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, the device comprising: a first heat exchanger configured to cool the feed gas stream to a temperature of between 110 K and 200 K in counter-current to a nitrogen vapour stream to produce a cooled stream, a second heat exchanger immersed in a liquid nitrogen bath configured to vaporise the liquid nitrogen by heat exchange with the cooled stream to produce nitrogen vapour, a distillation column configured to distil the cooled stream and produce a liquid containing at least 99.5 mol % of methane from the bottom of the column, a column boiler configured to receive the cooled stream before it is introduced into the distillation column, a condenser comprising a plate heat exchanger immersed in liquid nitrogen, the condenser being configured to cool a vapour phase withdrawn at the top of the distillation column by heat exchange with the liquid nitrogen, to form a gas enriched in nitrogen with respect to the vapour phase and a liquid enriched in methane with respect to the vapour phase, and to produce nitrogen vapour, at least one first conduit configured to send the cooled stream leaving the first heat exchanger to the second heat exchanger, at least one second conduit configured to send the nitrogen vapour leaving the second heat exchanger to the first heat exchanger, at least one third conduit configured to send the cooled stream leaving the second heat exchanger to the boiler, at least one fourth conduit configured to send the cooled stream leaving the boiler to the distillation column, at least one fifth conduit configured to withdraw the vapour phase from the top of the distillation column and to send the withdrawn vapor phase 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 vapour leaving the condenser to the first heat exchanger, and at least one eighth conduit configured to withdraw the liquid containing at least 99.5 mol % of methane from the bottom of the column as the final product.

10. The device according to claim 9 comprising a third heat exchanger immersed in a liquid nitrogen bath configured to cool the liquid withdrawn from the bottom of the distillation column and vaporise the liquid nitrogen to produce nitrogen vapour, and at least one ninth conduit configured to send the nitrogen vapour produced to the first heat exchanger.

11. The device according to claim 9 comprising a liquid nitrogen storage connected to the second heat exchanger and/or the third heat exchanger and/or the condenser.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0047] Other features and advantages will become apparent from the description below, with reference to the figure.

[0048] FIG. 1 schematically illustrates an example of an device and a process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0049] A gas stream 1 is fed into the device 100 to produce liquid methane 13 purified of nitrogen. The feed gas stream 1 contains at least 95 mol % of methane and up to 5 mol % of nitrogen. In particular, the feed gas stream 1 contains at least 98 mol % of methane and between 0.1 and 2 mol % of nitrogen. More particularly, the feed gas stream 1 contains at least 99 mol % of methane and between 0.1 and 1 mol % of nitrogen. Even more particularly, the feed gas stream 1 contains at least 99.5 mol % of methane and between 0.1 and 0.5 mol % of nitrogen.

[0050] The feed gas stream 1 is, for example, biomethane produced from biogas purification, for example by membrane separation, by washing 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 (temperature and/or pressure swing adsorption).

[0051] The feed gas stream 1 may be natural gas or a mixture of natural gas and biomethane.

[0052] The feed gas stream 1 is cooled in a first heat exchanger 3 in counter-current to a stream of nitrogen vapour 9. The first heat exchanger 3 is, for example, of the Brazed Aluminium Heat Exchangers (BAHX) type.

[0053] The cooled stream 5 leaving the first heat exchanger 3 is at a temperature of between 110 K and 200 K, in particular between 130 K and 150 K.

[0054] The cooled stream 5 leaving the first heat exchanger 3 passes through a second heat exchanger 7 which is, for example, immersed in a bath of liquid nitrogen, in which the cooled stream 5 and the bath of liquid nitrogen are in heat exchange. The cooled stream 5 heats the liquid nitrogen bath in the second heat exchanger 7 by heat exchange. This step lowers the temperature of the cooled gas stream 5 by approximately 1 K, thus enabling part of the liquid nitrogen to be vaporised and incorporated in vapour form into the first heat exchanger 3, while making the most of the latent heat of vaporisation. The nitrogen vapour 9 produced is returned to the first heat exchanger 3 to cool the feed gas stream 1 by heat exchange.

[0055] The use of a nitrogen bath avoids two-phase heat exchangers and direct contact between the liquid nitrogen and the gas stream 5.

[0056] The level of the bath is kept constant by adding liquid nitrogen from source 29 to compensate for the loss of level due to vaporisation.

[0057] The cooled stream 5 leaving 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 desired operating conditions, in particular in the upper part of the distillation column 11.

[0058] Preferably, before being introduced into the distillation column 11, the cooled stream 5 leaving the second heat exchanger 7 passes through a boiler 15 in the column. The cooled stream 5 recovers cold power to finish cooling, while delivering hot power to the boiler 15, by heat exchange in the boiler 15. The boiler 15 in the column may be located in the bottom of the column 11 or outside the column 11.

[0059] As illustrated, a vapour phase 17 depleted in methane and enriched in nitrogen compared with the feed gas stream 1, 5 is drawn off at the top of the distillation column 11. This vapour phase 17 may be sent to a condenser 19, which is separate from the distillation column 11, for example. Condenser 19 may comprise or may consist of a plate heat exchanger bathed in liquid nitrogen 21. The vapour 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 part of the vapour phase 17, forming a liquid 23 enriched in methane and depleted in nitrogen relative to the vapour phase 17 and a gas 22 enriched in nitrogen and depleted in methane relative to the vapour phase 17.

[0060] The methane-enriched liquid 23 is sent to the top of column 11 to form a reflux liquid. The nitrogen-enriched gas 22 is evacuated to a vent line, where the residual methane and/or the residual cold power can be recovered.

[0061] In condenser 19, liquid nitrogen 21 is vaporised 9 and fed back into the first heat exchanger 3 to cool the feed gas stream 1 by heat exchange.

[0062] 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 drawn off in the tank or at the bottom of the distillation column 11 as the final product. The liquid 13 withdrawn 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 bathed in a liquid nitrogen bath. In particular, the liquid 13 is in the saturated liquid state at a temperature of about 130 K. Its temperature is lowered to approximately 110 K via the third heat exchanger 25. Cooling in the third heat exchanger 25 prevents the liquefied methane from vaporising. The liquid nitrogen 21 vaporised 9 in this way can be reused in the first heat exchanger 3 to cool the feed gas stream 1 by heat exchange.

[0063] 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 frame in the figure). The device 100, in particular the heat exchangers 3, 7, 19, 25 and the distillation column 11, is preferably kept in the cold box.

[0064] The liquid nitrogen 21 used in the process according to the invention is preferably pressurised in order to avoid a temperature which would cause the methane to freeze.

[0065] 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 stream of refrigerant (nitrogen) vapour in the same (pre-)cooling exchanger (the first heat exchanger 3).

[0066] It is possible to achieve a methane molecule recovery rate of over 99.9%, while minimising refrigerant consumption.

[0067] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0068] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0069] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing i.e. anything else may be additionally included and remain within the scope of comprising. Comprising is defined herein as necessarily encompassing the more limited transitional terms consisting essentially of and consisting of; comprising may therefore be replaced by consisting essentially of or consisting of and remain within the expressly defined scope of comprising.

[0070] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0071] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0072] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0073] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.