METHOD FOR THE SEPARATION AND LIQUEFACTION OF METHANE AND CARBON DIOXIDE WITH REMOVAL OF THE AIR IMPURITIES PRESENT IN THE METHANE

Abstract

A combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, including a mixing means, a compressor, a first exchanger, a distillation column, a second exchanger, a separating means, an expanding means, and a separator vessel. Wherein, the mixing means is configured such that the recycle gas is the overhead vapour stream, and the first exchanger and the expanding means are combined.

Claims

1. A combined facility for the cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, comprising: a mixing means for mixing a biogas stream with a recycle gas stream, thereby producing a mixed biogas stream, a compressor for compressing the mixed biogas stream to a pressure suitable for distillation, thereby producing a compressed stream, a first exchanger for cooling the compressed stream, a distillation column, comprising a top and a bottom, and configured to be supplied with the cooled mixture and configured to produce a methane-enriched stream at the top and a first CO.sub.2-enriched liquid stream at the bottom, a second exchanger for liquefying the methane-enriched stream, thereby producing a liquefied methane stream, a separator vessel for receiving the liquefied methane stream and for recovering an overhead vapour stream and liquid methane stream, a pumping means for sending the liquid methane to the top, a distillation column supplied with the overhead vapour stream and configured to produce an impurity enriched stream at the top and a methane-enriched liquid stream at the bottom, an expanding means for expanding and heating the first CO.sub.2-enriched liquid stream and for recovering the cold from the first CO.sub.2-enriched liquid stream, thereby producing a first heated CO.sub.2-enriched stream, and a first separator vessel for receiving the first heated CO.sub.2-enriched stream and for recovering a first overhead vapour stream and a second CO.sub.2-enriched liquid stream, wherein the mixing means is configured such that the recycle gas stream is the overhead vapour, and the first exchanger and the expanding means are combined.

2. The facility according to claim 1, further comprising a heating means for heating the second CO.sub.2-enriched liquid stream, thereby producing a second heated CO.sub.2-enriched stream, and a second separator vessel for receiving second heated CO.sub.2-enriched stream and for recovering a second overhead vapour stream and liquid CO.sub.2 stream.

3. The facility according to claim 1, further comprising, upstream of the mixing means, a drying means for drying and desulfurization of the biogas stream.

4. The facility according to claim 1, further comprising, upstream of the mixing means, a compressing means for compressing the biogas stream to the pressure of the recycle gas stream.

5. The facility according to claim 1, further comprising, upstream of the mixing means, a cooling means for cooling the biogas stream to ambient temperature.

6. The facility according to claim 1, wherein the second exchanger is within a closed refrigeration circuit.

7. The facility according to claim 6, wherein the refrigeration circuit uses methane as refrigerant fluid.

8. The facility according to claim 1, wherein the distillation column comprises heating at the bottom.

9. A combined process for the cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, using the facility as defined in claim 1, the process comprising: a) mixing the biogas stream with a recycle gas stream, b) compressing the mixed biogas stream to the distillation pressure, c) cooling the compressed mixed biogas stream in the first exchanger, d) distilling the cooled stream in the distillation column thereby producing the methane-enriched stream at the top and the first CO.sub.2-enriched liquid stream at the bottom, e) liquefying the methane-enriched stream in the second exchanger, f) separating the liquefied methane stream in the separator vessel into the liquid methane stream and the overhead vapour stream, g) sending the liquid methane stream to the top of the distillation column, h) distilling the overhead vapour stream in the distillation column K02 thereby producing an impurity rich stream at the top and a methane-enriched liquid stream at the bottom, i) expanding and heating the first CO.sub.2-enriched liquid stream, and of recovering the cold from the first CO.sub.2-enriched liquid stream, and j) a step of separating the first CO.sub.2-enriched stream in the separator vessel into the liquid CO.sub.2 stream and the overhead vapour stream, with the recycle gas stream corresponding to the overhead vapour stream produced in step a).

10. The process according to claim 9, further comprising heating the first CO.sub.2-enriched liquid and a step of separating the first heated CO.sub.2-enriched liquid in the second separator vessel into the overhead vapour stream and the second liquid CO.sub.2 stream.

11. The process according to claim 9, further comprising, upstream of step a), steps of drying and of desulfurization of the biogas stream.

12. The process according to claim 9, further comprising, upstream of step a), a step of compressing the biogas stream to the pressure of the recycle gas stream.

13. The process according to claim 9, further comprising, upstream of step a), a step of cooling the biogas stream to ambient temperature.

14. The process according to claim 9, further comprising, downstream of step j), a step of heating and vaporizing the liquid CO.sub.2 stream

15. The process according to claim 9, step e) is performed by cooling the methane stream by means of a refrigerant fluid.

16. The process according to claim 9, in step b), the mixed biogas stream is compressed to a pressure of between 7 and 46 bar.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0056] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0057] FIG. 1 illustrates a refrigeration circuit in accordance with one embodiment of the present invention

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0058] The pretreated biogas 1 (pretreated by drying, desulfurization) is introduced into the process at atmospheric pressure and temperature, it is compressed a first time in a compressor C01, to the pressure of the recycle circuit (around 8 bar). After compression, it is cooled in C01E to ambient temperature with CW (=Cooling Water) or air.

[0059] Next, it is mixed with a recycle stream R, the mixture is compressed in a compressor CO.sub.2, to the pressure of the distillation column (around 15 bar) or more depending on the requirements of the downstream exchanger E01 and it is cooled to ambient temperature in C02E, with CW or air.

[0060] Preferably, C01E and C02E are shell and tube exchangers (coolers of the compressors)

[0061] The mixture of biogas—recycle stream R is sent to the exchanger E01. The main purpose of this exchanger is to cool the mixture in preparation for the distillation.

[0062] The mixture can then be expanded or supplied directly to the column where it will be used as reboiler.

[0063] If there is no heat source at the bottom of the column, it is necessary to inject the mixture into the bottom to ensure the circulation of vapour from the bottom. If there is a heat source in the bottom of the column (reboiler), the mixture is introduced higher up in the column.

[0064] The distillation column K01 separates the methane from the carbon dioxide. The feed for the column is the biogas+recycle stream R mixture. This feed acts as main reboiler; an additional source of heat may also be used (for example an electrical resistance heater, vapour or a portion of the hot biogas in indirect contact). The product at the top of the column is pure CH.sub.4 in the vapour state. The bottom product is a liquid rich in CO.sub.2, containing around 95%-98%.

[0065] The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit. The methane-enriched liquid 2 resulting from the exchanger E02 is separated in the separator vessel V04 into liquid methane and overhead vapour. Typically, the overhead vapour will comprise methane, oxygen and nitrogen. The liquid methane 3 from the separator vessel V04 is used as column recycle and is fed back into the top of the distillation column K01. The overhead vapour obtained from the separator vessel V04 is distilled in the distillation column K02 so as to produce a stream enriched in impurities (oxygen and nitrogen) at the top of the column K02 and a methane-enriched liquid 4 at the bottom of the column K02. Preferably, the overhead vapour from the separator vessel V04 is introduced into the distillation column at an intermediate stage of the column.

[0066] The CO.sub.2-enriched liquid recovered at the bottom of the column is expanded and heated in the exchanger E01 countercurrent to the biogas—recycle stream R mixture.

[0067] The CO.sub.2-enriched stream from the exchanger E01 is sent to the separator vessel V01.

[0068] The overhead vapour of the vessel V01 is reheated in the exchanger E01 and then mixed with the biogas. It corresponds to the stream previously named “recycle stream R”.

[0069] The liquid from the bottom of the vessel V01 is the pure CO.sub.2 5. This can, depending on the requirements, leave the process as product or be reheated in the exchanger E01 and in another exchanger E03 of the refrigeration circuit in order to be completely vaporized before leaving the cycle. Note that the pure CO.sub.2 could alternatively be reheated and vaporised in the exchanger E03 without passing through the exchanger E01.

[0070] Preferentially, the pure liquid CO.sub.2 (CO.sub.2-enriched liquid) leaving the separator vessel V01 will be heated in the exchanger E01 and then introduced into a separator vessel V03 to be separated into an overhead vapour and liquid CO.sub.2 which is even purer than that leaving the separator vessel V01.

[0071] The exchanger E01 thus uses, as sources of cold: the CO.sub.2-enriched liquid recovered at the bottom of the column, the overhead vapour V01 called the “recycle stream R” at the outlet of the exchanger E01, and optionally the pure liquid CO.sub.2 recovered at the bottom of the vessel V01 in the case where its vaporization is desired or if it is desired to purify it even further in a separator vessel V03.

[0072] The process requires an input of refrigeration power in order to operate. This input of cold is represented in FIG. 1 by the refrigeration circuit. It is composed of: [0073] a compressor C03 with cooler C03E; [0074] an exchanger E03 which cools the compressed fluid using the recycled refrigerant fluid and the cold recovered from the separation cycle; [0075] a turbine ET01 and a JT valve PV05, for the expansion of the refrigerant fluid and production of cold; [0076] a separator vessel V02 for separating the vapour and liquid phases of the refrigerant fluid; [0077] an exchanger E02 which uses the liquid phase of the refrigerant fluid to liquefy the biomethane at the top of the distillation column. [0078] The refrigerant fluid used in the scheme is CH.sub.4, but it can be replaced with other fluids such as N.sub.2, N.sub.2+H.sub.2, inter alia.

[0079] This refrigeration cycle can be replaced by other sources of cold (depending on the amount of liquid biomethane to be produced). By way of example but not exclusively: [0080] using a source of liquid nitrogen; [0081] by a Brayton cycle process.

[0082] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.