PROCESS FOR SEPARATING AND LIQUEFYING METHANE AND CO2 COMPRISING THE WITHDRAWAL OF VAPOUR FROM AN INTERMEDIATE STAGE OF THE DISTILLATION COLUMN

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 plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas flow, 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, an 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 stream at the top and a CO.sub.2-enriched liquid stream at the bottom, a second exchanger for liquefying the methane stream, thereby producing a liquefied methane stream a separating means for separating the liquefied methane stream into two portions: a “reflux” portion and a “product” portion, an expanding means for expanding and heating the CO.sub.2-enriched liquid stream and for recovering the cold from the CO.sub.2-enriched liquid stream, thereby producing a heated CO.sub.2-enriched stream, and a separator vessel for receiving the heated CO.sub.2-enriched stream and for recovering an overhead vapour stream and a liquid CO.sub.2 stream, a withdrawing means for withdrawing vapour from an intermediate stage of the distillation column, a condensing means for partially condensing the vapour and producing a two-phase flow, a reinjecting means for reinjecting the two-phase flow into the distillation column at the stage corresponding to an equilibrium temperature, 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 plant according to claim 1, wherein the condensing means is the first exchanger.

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

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

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

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

8. The plant according to claim 1, wherein the distillation column comprises a heater at the bottom.

9. A combined process of cryogenic separation and liquefaction of methane and carbon dioxide in a biogas flow, using the plant as defined in claim 1, the process comprising: a) mixing the biogas stream with the 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 stream and the CO.sub.2-enriched liquid stream, e) liquefying the methane in the second exchanger, f) separating the liquefied methane stream into the “reflux” portion and the “product” portion, g) expanding and heating the CO.sub.2-enriched liquid stream, and recovering the cold from the CO.sub.2-enriched liquid stream, and h) separating the heated CO.sub.2-enriched stream in the separator vessel into a liquid CO.sub.2 stream and an overhead vapour stream, with the recycle gas stream corresponding to the overhead vapour produced in step a), and the distillation step d) comprising the following sub-steps: i) withdrawing the vapour from an intermediate stage of the distillation column, ii) cooling the vapour in the first exchanger thereby partially condensing the vapour and producing a two-phase stream, iii) reinjecting the two-phase stream into the distillation column at the stage corresponding to an equilibrium temperature.

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

11. 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.

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

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

14. The process according to claim 13, wherein the heating of the liquid CO.sub.2 stream is carried out in the first exchanger.

15. The process according to claim 9, wherein step e) is carried out by cooling the methane steam by means of a refrigerant fluid.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] 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:

[0058] FIG. 1 illustrates a refrigeration circuit in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0059] 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.

[0060] Next, it is mixed with a recycle flow 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.

[0061] Preferably, C01E and C02E are shell and tube exchangers (coolers of the compressors)

[0062] The mixture of biogas—recycle flow R is sent to the exchanger E01. The main objective of this exchanger is to cool the mixture in preparation for the distillation. 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 flow 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] Vapour V is withdrawn from an intermediate stage of the distillation column K01. This vapour V is sent into the exchanger E01 and partially condensed so as to produce a two-phase flow D. The two-phase flow D is reinjected into the column at the stage corresponding to the equilibrium temperature.

[0066] The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit. A portion 2 of the methane leaves the cycle as product and the other portion 3 (reflux portion) is used as recycle for the column and reinjected at the top of the column.

[0067] 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 flow R mixture.

[0068] The CO.sub.2-enriched flow from the exchanger E01 is sent to the separator vessel V01.

[0069] The overhead vapour of the vessel V01 is reheated in the exchanger E01 and then mixed with the biogas. It corresponds to the flow previously named “recycle flow R”.

[0070] The liquid from the bottom of the vessel V01 is the pure CO.sub.2 4. 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.

[0071] The exchanger E01 therefore uses, as sources of cold: the CO.sub.2-enriched liquid recovered at the bottom of the column, the overhead vapour from the vessel V01 named “recycle flow R” at the outlet of the exchanger E01, and optionally the liquid pure CO.sub.2 recovered at the bottom of the vessel V01 in the case where the vaporisation thereof is desired.

[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 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 by other fluids such as N2, N2+H2, 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.