PROCESS FOR LIQUEFYING NATURAL GAS AND CARBON DIOXIDE

Abstract

A process for producing liquefied natural gas (14) and liquid carbon dioxide (CO.sub.2) (15) comprising at least the following steps: Step a): separating a natural gas feed gas (1), containing hydrocarbons and carbon dioxide in a treatment unit (2), into a CO.sub.2-enriched gas stream (4) and a CO.sub.2-depleted natural gas stream (3); step b): liquefying the CO.sub.2-depleted natural gas stream (3) resulting from step a) in a natural gas liquefaction unit (5) comprising at least a main heat exchanger (8) and a system (9) for producing frigories; step c): simultaneous liquefying of the CO.sub.2-enriched gas stream (4) resulting from step a) in a CO.sub.2 liquefaction unit (6); characterized in that all of the refrigeration necessary for the liquefaction of the CO.sub.2-enriched gas stream (4) and for the liquefaction of the natural gas is supplied by said frigorie-producing system (9) of the natural gas liquefaction unit (5).

Claims

1. A process for producing liquefied natural gas) and liquid carbon dioxide (CO.sub.2) comprising the following steps: Step a): separating a natural gas feed gas, containing hydrocarbons and carbon dioxide in a treatment unit, into a CO.sub.2-enriched gas stream and a CO.sub.2-depleted natural gas stream; Step b): liquefying the CO.sub.2-depleted natural gas stream resulting from step a) in a natural gas liquefaction unit comprising at least a main heat exchanger and a system for producing frigories; Step c): simultaneous liquefying of the CO.sub.2-enriched gas stream resulting from step a) in a CO.sub.2 liquefaction unit; wherein all or some of the refrigeration necessary for the liquefaction of the CO.sub.2-enriched gas stream and for the liquefaction of the natural gas is supplied by said frigorie-producing system of the natural gas liquefaction unit); and wherein the CO.sub.2-enriched gas stream resulting from step a) is purified prior to step c), the refrigeration necessary for this purification being provided by said frigorie-producing system of the natural gas liquefaction unit.

2. The process according to claim 1, wherein the feed gas comprises from 0.1 mol % to 5 mol % of CO.sub.2.

3. The process according to claim 1, wherein the CO.sub.2-enriched gas stream resulting from step a) comprises at least 95 mol % of CO.sub.2.

4. The process according to claim 1, wherein prior to step b), the natural gas stream resulting from step a) is pretreated in a pretreatment unit.

5. The process according to claim 1, wherein said treatment unit used in step a) is an amine scrubbing unit.

6. The process according to claim 5, wherein the CO.sub.2-enriched stream thus purified comprises at least 99.5 mol % of CO.sub.2.

7. The process according to claim 1, wherein said system for producing frigories comprises at least one compressor.

8. The process according to claim 1, wherein said natural gas liquefaction unit comprises at least one refrigeration cycle fed by a refrigerant stream containing at least one of the constituents selected from the group consisting of nitrogen, methane, ethylene, ethane, propane, ammonia, butane and pentane.

9. The process according to claim 1, wherein the refrigeration necessary for the liquefaction of the CO.sub.2-enriched gas stream originates from a refrigerant fluid precooled in the heat exchanger of the natural gas liquefaction unit.

10. The process according to claim 9, wherein the refrigerant fluid is an ammonia and/or propane fluid.

11. A device for producing liquefied natural gas and liquefied CO.sub.2 comprising a feed gas treatment unit, producing at least a CO.sub.2-enriched gas stream and a CO.sub.2-depleted natural gas stream, and a natural gas liquefaction unit, said natural gas liquefaction unit comprising at least a main heat exchanger and a system for producing frigories, wherein the system for producing frigories is capable of and designed for liquefying both the CO.sub.2-enriched stream resulting from the treatment unit and the natural gas stream circulating in the natural gas liquefaction unit; and further comprising a CO.sub.2-enriched gas purification and liquefaction unit comprising at least one compression means, one purification means and at least one distillation column, wherein said device is designed so that the refrigeration necessary for the use of the CO.sub.2-enriched gas purification and liquefaction unit originates partly or completely from a refrigerant fluid cooled in said main heat exchanger of the natural gas liquefaction unit.

12. The device according to the claim 11, wherein said system for producing frigories comprises at least one compressor.

13. The device according to claim 12, wherein said system for producing frigories comprises a refrigeration cycle, comprising a compressor driven by an electric or heat engine, a circuit for circulating a refrigerant fluid.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0061] 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 drawing, in which like elements are given the same or analogous reference numbers and wherein:

[0062] FIG. 1 illustrates a schematic representation of one embodiment of present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0063] In FIG. 1, a natural gas feed stream 1 (for example at a considered flow rate of 500 000 tonnes per year, i.e. around 60 tonnes per hour) containing CO.sub.2 is introduced into a treatment unit 2 in which said stream 1 is separated into at least two gas streams 3 and 4. The natural gas feed stream 1 contains for example from 0.1 mol % to 5 mol % of CO.sub.2.

[0064] The first stream 3 is a CO.sub.2-depleted natural gas stream. The second stream 4 is a CO.sub.2-enriched stream.

[0065] The treatment unit 2 is a unit that separates the CO.sub.2 from the natural gas stream, for example a chemical absorption unit, in particular an amine (of MDEA, MEA, etc. type) scrubbing unit that makes it possible to produce concentrated CO.sub.2 at low pressure (typically slightly greater than atmospheric pressure). Concentrated (or purified) CO.sub.2 is understood to mean a stream containing more than 95 mol % of CO.sub.2 on a dry basis.

[0066] After possible pre-treatment steps to eliminate all traces of mercury, water or sulphur derivatives for example (unit 7), the CO.sub.2-depleted natural gas stream 3 is introduced into the main exchanger 8 of a natural gas liquefaction unit 5 in order to be liquefied 14. A stream 10 rich in heavy hydrocarbons is extracted from the liquefaction unit 5 in order to avoid any risk of freezing. Said stream 10 being extracted from the liquefaction unit 5 via a scrubbing column 11. A refrigerant stream circulates in a closed cycle in the heat exchanger 8 in order to provide the refrigeration necessary for liquefying said natural gas stream 3.

[0067] In particular, the liquefaction cycle 9 uses a refrigerant that may be a mixture of refrigerants typically selected from nitrogen, methane, ethane, ethylene, propane, butane and pentane. It may be a cycle based on a refrigeration cycle consisting of a refrigerant or a mixture of several refrigerants.

[0068] A refrigerant stream (not shown in FIG. 1) is introduced into the frigorie-producing system 9 of the liquefaction unit 5 via a compressor (and optionally via a compressor/booster system).

[0069] The second CO.sub.2-enriched gas stream 4 resulting from the treatment unit 2 is compressed to medium pressure (typically 25 bar abs), cooled, purified (elimination of any trace of H.sub.2O, hydrocarbons, sulphur derivatives in particular) then sent back to a distillation column (stripping column) that separates the noncondensable gases at the top from the concentrated liquid CO.sub.2 15 recovered at the bottom.

[0070] In order to provide the refrigeration necessary for the correct operation of the purification/liquefaction unit 6, a refrigerant fluid 13 (typically but not only ammonia or propane) circulates between the CO.sub.2 purification/liquefaction unit 6 and the natural gas liquefaction unit 5: it enters the heat exchanger 8 at ambient temperature, is condensed at a temperature between −10° C. and −40° C. (typically around −20° C./−30° C.) in order to be sent back to the CO.sub.2 purification/liquefaction unit 6.

[0071] Thus, owing to this thermal integration, a refrigeration cycle dedicated to the CO.sub.2 purification/liquefaction unit 6 is avoided by increasing the power of the cycle dedicated to the natural gas liquefaction (typically by the order of 5%).

[0072] 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.