PROCESS FOR LIQUEFYING CARBON DIOXIDE RESULTING FROM A NATURAL GAS STREAM

Abstract

A process for producing liquefied natural gas and liquid carbon dioxide comprising: Step a): separating a natural gas feed gas 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 in a liquefaction unit comprising at least a main heat exchanger and a system for producing frigories, said liquefaction unit comprising at least one refrigeration cycle fed by a refrigerant stream; Step c): simultaneous liquefying of the CO.sub.2-enriched gas stream resulting from step a) in a CO.sub.2 liquefaction unit; wherein 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 liquefaction unit and in that the refrigeration necessary for the liquefaction of the CO.sub.2-enriched gas stream originates from a portion of said refrigerant stream supplying the refrigeration cycle of said liquefaction unit.

Claims

1. A process for producing liquefied natural gas and liquid carbon dioxide comprising at least 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, said natural gas liquefaction unit comprising at least one refrigeration cycle fed by a refrigerant stream; Step c): simultaneous liquefying of the CO.sub.2-enriched gas stream resulting from step a) in a CO.sub.2 liquefaction unit; wherein 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 in that the refrigeration necessary for the liquefaction of the CO.sub.2-enriched gas stream originates from a portion of said refrigerant stream supplying the refrigeration cycle of said natural gas liquefaction unit; and wherein the feed gas comprises from 0.1 mol % to 5 mol % of CO.sub.2.

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

3. 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.

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

5. The process according to claim 1, 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.

6. The process according to claim 1, 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 refrigerant stream supplying said at least one refrigeration cycle of said natural gas liquefaction unit contains at least one of the constituents selected from the group consisting of nitrogen, methane, ethylene, ethane, propane, ammonia, butane and pentane.

Description

BRIEF DESCRIPTION OF THE DRAWING

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

[0058] FIG. 1 illustrates a schematic representation of one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0059] In FIG. 1, a natural gas feed stream 1 (flow rate considered: 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.

[0060] The first stream 3 is a CO.sub.2-depleted natural gas stream. The second stream 4 is a CO.sub.2-enriched stream.

[0061] 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 pure (or concentrated) CO.sub.2 at low pressure (typically slightly greater than atmospheric pressure). Pure CO.sub.2 is understood to mean a stream containing more than 95 mol % of CO.sub.2 on a dry basis.

[0062] After possible pre-treatment steps to eliminate all traces of mercury, water or sulphur derivatives for example (pre-treatment in 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.

[0063] The pressure of this gas stream is for example between 25 and 60 bar absolute. Typically, the gas stream 3 contains between 30 ppm by volume and 500 ppm by volume of benzene, usually less than 100 ppm by volume. The gas stream 3 is cooled by heat exchange in the heat exchanger 8 in contact with a refrigerant. The heat exchanger 8 is supplied by at least one refrigerant stream 8.

[0064] For example, this stream may be composed of a nitrogen or mixed refrigerant stream that provides the refrigeration necessary for the liquefaction of the natural gas stream. The refrigerant stream is sent into the exchanger at high pressure (typically from 30 to 60 bar) and sent back at low pressure (from 1 to 10 bar). The recompression energy necessary for the operation of the refrigeration cycle is provided by a cycle compressor (optionally supplemented by a turbine/booster system within the context of a nitrogen cycle).

[0065] The CO.sub.2-depleted natural gas stream 3 introduced into the main exchanger 8 of a natural gas liquefaction unit 5 is for example liquefied according to the process described in the following lines.

[0066] The natural gas stream cooled to a temperature between −20° C. and −70° C., typically between −35° C. and −40° C. at the outlet of the exchanger 8 is introduced into a unit 11 for separating the heavy hydrocarbons from the natural gas stream, for example a scrubbing column in which the heavy products 10 are separated from the natural gas. Heavy products are understood to mean hydrocarbons having more than four carbon atoms and aromatic compounds including in particular benzene.

[0067] A liquid stream 10 containing all the hydrocarbons that it is desired to extract from the natural gas stream, such as benzene, (from the initial gas stream 1), is discharged at the bottom of the scrubbing column.

[0068] At the top of the column, a gas stream that no longer presents a risk of freezing due to the presence of heavy hydrocarbons or aromatic derivatives (typically comprising less than 1 ppm by volume of benzene) is recovered in order to be introduced into a second section of the heat exchanger 8. Via heat exchange, it is cooled to the desired temperature (typically −160° C.) in order to be sent to a liquefied natural gas storage means 14.

[0069] The mixed refrigerant stream recovered at the outlet of the heat exchanger 8 is introduced into a phase separator vessel that produces a gas stream containing the light elements of the refrigerant at the top of the vessel and a liquid stream 13 containing the heavy elements of the refrigerant at the bottom of the vessel. The 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.

[0070] 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.

[0071] A refrigerant stream is introduced into the 9 rigories-producing system 9 of the liquefaction unit 5 via a compressor (and optionally via a compressor/booster system).

[0072] 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.

[0073] In order to provide the refrigeration necessary for the correct operation of the purification/liquefaction unit 6, a portion of the liquid stream 13 containing the heavy elements of the refrigerant is extracted and is sent to circulate between the CO.sub.2 purification/liquefaction unit 6 and the natural gas liquefaction unit 5. 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%).

[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.