COMBINED PLANT FOR CRYOGENIC SEPARATION AND LIQUEFACTION OF METHANE AND CARBON DIOXIDE COMPRISED IN A BIOGAS STREAM

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 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 ae pressure suitable for distillation, thereby producing a compressed stream, a first exchanger for cooling the compressed stream, thereby producing a cooled stream, a distillation column, comprising a top and a bottom, and configured to be supplied with the cooled stream 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 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 CO.sub.2-enriched stream and for recovering an overhead vapour stream and liquid CO.sub.2 stream, 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.

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

3. 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.

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

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

6. The plant according to claim 5, wherein the refrigeration circuit uses methane as refrigerant fluid.

7. The plant according to claim 1, wherein the distillation column comprises heating at the bottom.

8. A combined process of cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, using the plant as defined in claim 1, and 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 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 stream 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 in the first exchanger, 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 V01 into the liquid CO.sub.2 stream and the overhead vapour stream, with the recycle gas stream is the overhead vapour produced in step a).

9. The process according to claim 8, further comprising, upstream of step a), drying and of desulfurization the biogas stream.

10. The process according to claim 8 further comprising, upstream of step a), a step of compressing the biogas stream to the pressure of the recycle gas stream.

11. The process according to claim 8, further comprising, upstream of step a), a step of cooling the biogas stream to ambient temperature.

12. The process according to claim 8, further comprising, downstream of step h), a step of heating and vaporizing the liquid CO.sub.2 stream.

13. The process according to claim 8, wherein step e) is performed by cooling the methane stream by means of a refrigerant fluid.

14. The process according to claim 8, wherein, in step b), the mixed biogas stream is compressed to a pressure of between 7 and 46 bar.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0050] 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:

[0051] FIG. 1 illustrates a refrigeration circuit in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0052] 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.

[0053] 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 CO.sub.2E, with CW or air.

[0054] Preferably, C01E and CO.sub.2E are shell and tube exchangers (cooler of the compressors).

[0055] 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. The mixture can then be expanded or supplied directly to the column where it will be used as reboiler.

[0056] 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.

[0057] 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%.

[0058] The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit. A portion of the methane leaves the cycle as product 2 and the other portion 3 (reflux portion) is used as recycle for the column and reinjected into the top of the column.

[0059] 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.

[0060] The CO.sub.2-enriched stream from the exchanger E01 is sent to the separator vessel V01.

[0061] 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”.

[0062] 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.

[0063] 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 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 the vaporisation thereof is desired.

[0064] 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: [0065] a compressor C03 with cooler C03E; [0066] an exchanger E03 which cools the compressed fluid using the recycled refrigerant fluid and the cold recovered from the separation cycle; [0067] a turbine ET01 and a JT valve PV05, for the expansion of the refrigerant fluid and production of cold; [0068] a separator vessel V02 separating the vapour and liquid phases of the refrigerant fluid; [0069] an exchanger E02 which uses the liquid phase of the refrigerant fluid to liquefy the biomethane at the top of the distillation column. [0070] The refrigerant fluid used in the scheme is CH.sub.4 but it can be replaced by other fluids such as N.sub.2, N.sub.2+H.sub.2, inter alia.

[0071] 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: [0072] using a source of liquid nitrogen; [0073] by a Brayton cycle process.

[0074] 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.