PROCESS FOR PURIFYING NATURAL GAS AND LIQUEFYING CARBON DIOXIDE

Abstract

A process for producing liquefied natural gas and liquid carbon dioxide from a natural gas feed gas comprising at least the following steps: Separation of a natural gas feed gas into a CO.sub.2-enriched gas stream and a natural gas stream; Cooling of said natural gas in a heat exchanger; Purification of the in step 1 from compounds containing at least six carbon atoms; At least partial condensation of said gas stream resulting from step 3 to form a two-phase stream; Separation of said two-phase stream resulting from step 4 to form a gas stream and a liquid stream; Condensation of the gas stream resulting from step 5 to form a liquefied gas containing less than 5 ppm by volume of compounds containing at least six carbon atoms; Liquefaction of the CO.sub.2-enriched gas stream resulting from step 1 with a portion of the liquid stream resulting from step 5.

Claims

1. A process for producing liquefied natural gas and liquid carbon dioxide (CO.sub.2) from a natural gas feed gas that contains carbon dioxide and that is rich in hydrocarbons comprising at least 10 ppm by mole of hydrocarbons having at least six carbon atoms, comprising at least the following steps: Step a): Separation of a natural gas feed gas, containing hydrocarbons and carbon dioxide in a treatment unit, into a CO.sub.2-enriched gas stream and a natural gas stream; Step b): Cooling of said natural gas to a temperature between −20° C. and −60° C. by heat exchange with at least one refrigerant in a heat exchanger; Step c): Purification of the gas partially liquefied in step a) from compounds containing at least six carbon atoms in a scrubbing column containing a top of the column in its highest end and a bottom of the column in its lowest end in order to form, at the top of the scrubbing column, a gas stream containing less than 5 ppm by volume of compounds containing at least six carbon atoms, and, at the bottom of the scrubbing column, a liquid stream enriched in compounds containing at least five carbon atoms; Step d): At least partial condensation of said gas stream resulting from step c) in a heat exchanger in order to form a two-phase stream; Step e): Separation of said two-phase stream resulting from step d) in a phase separator vessel at a temperature between −60° C. and −80° C. in order to form a gas stream at the top of the vessel and a liquid stream at the bottom of the vessel; Step f): Use of a first portion of the liquid stream resulting from step e) as reflux at the top of the scrubbing column; Step g): Condensation of the gas stream resulting from step e) by heat exchange in a heat exchanger at a temperature below −100° C. in order to form a liquefied gas containing less than 5 ppm by volume of compounds containing at least six carbon atoms; Step h): Liquefaction of the CO.sub.2-enriched gas stream resulting from step a) by heat exchange with a second portion of the liquid stream resulting from step e).

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

3. The process of claim 1, wherein the gas stream resulting from step c) and the liquefied natural gas resulting from step d) contain less than 1 ppm by mole of compounds containing at least six carbon atoms.

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

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

6. The process of claim 1, wherein said hydrocarbons having at least six carbon atoms comprise a majority of benzene.

7. The process of claim 1, wherein the operating temperature of step b) is between −20° C. and −40° C.

8. The process of claim 1, wherein the second portion of the liquid stream resulting from step e), having been used to liquefy the CO.sub.2-enriched stream during step g), is then reinjected into the bottom of the scrubbing column.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0062] 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:

[0063] FIG. 1 illustrates a schematic representation of embodiment of one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0064] In FIG. 1, a natural gas feed stream 1 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.

[0065] The natural gas feed stream 1 contains for example from 0.1 mol % to 5 mol % of CO.sub.2.

[0066] The first stream 3 is a CO.sub.2-depleted natural gas stream. The second stream 4 is a CO.sub.2-enriched stream.

[0067] The treatment unit 2 is 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.

[0068] The CO.sub.2-depleted natural gas stream 3 is introduced into the main exchanger 2′ of a natural gas liquefaction unit in order to be liquefied.

[0069] 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 2′ in contact with a refrigerant. The heat exchanger 2′ is supplied by at least one refrigerant stream 3′. For example, this stream may be composed of a mixed refrigerant stream. The composition and the operating conditions of the mixed refrigerant are adjusted to the hydrocarbon to be liquefied.

[0070] The CO.sub.2-depleted natural gas stream 3 introduced into the main exchanger 2′ of a natural gas liquefaction unit is for example liquefied according to the process described in the following lines.

[0071] The natural gas stream cooled to a temperature between −20° C. and −70° C., typically between −35° C. and −40° C. at the outlet 4′ of the exchanger 2′ is introduced into a scrubbing column 5 in which the heavy products 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.

[0072] A liquid stream 6 containing all (to within 1 ppm by volume) of the benzene from the initial gas stream 1 is discharged at the bottom 7 of the column 5.

[0073] At the top 8 of the column 5, a gas stream 9 comprising less than 1 ppm by volume of benzene is recovered in order to be introduced into a second heat exchanger 10 which may preferentially be a second section of the heat exchanger 2′.

[0074] The mixed refrigerant stream 11 recovered at the outlet of the heat exchanger 2′ is introduced into a phase separator vessel 12 that produces a gas stream 13 containing the light elements of the refrigerant at the top of the vessel 12 and a liquid stream 14 containing the heavy elements of the refrigerant at the bottom of the vessel 12. These two streams 13 and 14 supply the second heat exchanger 10 (or second stage of the exchanger 2′).

[0075] The gas stream 9 containing less than 1 ppm by volume of benzene introduced into the second heat exchanger 10 (or second section of the exchanger 2) is at least partially condensed. The two-phase stream 15 at the outlet of the second heat exchanger 10 (or second section of the exchanger 2′) is introduced into a phase separator vessel 16 in order to produce a gas stream 17 at the top of the vessel 16 and a liquid stream 18 at the bottom of the vessel 16. The temperature is then typically between −70° C. and −75° C.

[0076] A first portion 18′ of the liquid stream 18 supplies the top 8 of the scrubbing column 5. Depending on the installation of the vessel 16 relative to the column 5, a pair 19 of lift pumps may be present in order to suck up the liquid stream 18′ in order to effect the reflux at the top 8 of the column 5.

[0077] It should be noted that the liquid reflux (stream 18′) may not be sufficient and that in this case, it is possible to cool the vessel 16 by injecting liquid natural gas at the inlet of the two-phase vessel (line 22). This line 22 is important since it makes it possible to control the flow rate of liquid reflux in the column 5 and thus the benzene content of the product to be liquefied. As the composition and the operating conditions of the natural gas may change over the lifetime of the unit, the reflux flow rate required may thus be optimized, and also the liquefaction energy.

[0078] There are at least two variants for this solution: [0079] Variant no. 1: In order to simplify the main exchanger, it is possible to directly “shower” the top of column 5 with liquid natural gas (stream 22) but the flow rate of liquid natural gas 22 is then greater and this option may be expensive in terms of liquefaction energy. [0080] Variant no. 2: In order to reduce the flow rate of liquid natural gas required, it is also possible to shower the top of the separator vessel 16 and thus to purify the natural gas via two successive refluxes.

[0081] The gas stream 17 is introduced into a third heat exchanger 20 which may preferentially be a third section of the exchanger 2′ in order to be cooled to a temperature below −110° C., for example between −110° C. and −115° C. The stream 21 thus cooled may be partly tapped and form a stream 22 that will be recycled by being introduced with the stream 15 into the phase separator vessel 16.

[0082] Indeed, according to one particular embodiment of the process for liquefying natural gas 1 that is the subject of the invention, the liquid refrigerants 14 are drawn off then, subsequently, expanded, for example with the aid of valves 23, before being reintroduced and re-vaporized in the exchange line 24 opposite the natural gas 17 being liquefied.

[0083] Thus, a stream of liquid natural gas may be tapped 22 at this level (colder than the phase separator vessel 16 forming the reflux of the column 5) and recycled to the vessel 16 in order to increase the reflux liquid 18′ in the event of lack of the latter, while limiting the loss of effectiveness by instantaneous vaporization.

[0084] Finally, the gas stream 21 is introduced into a heat exchanger 26 in order to produce a stream of liquefied natural gas 27, a product purified from heavy compounds and from aromatics, including typically benzene, resulting from the liquefaction and purification process that is the subject of the present invention.

[0085] The second CO.sub.2-enriched gas stream 4 resulting from the treatment unit 2 is liquefied in a CO.sub.2 purification/liquefaction unit 28 by heat exchange with a second portion 18″ of the liquid stream 18 resulting from the bottom of the separator vessel 16, used as refrigerant fluid.

[0086] The purification/liquefaction unit 28 typically comprises compression means, purification means and liquefaction means, for example a distillation column provided with a condensation means supplied with refrigerant fluid.

[0087] Thus, 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 H.sub.2O, heavy hydrocarbons, sulphur derivatives in particular) then sent back to a distillation column that separates the noncondensable gases at the top from the pure liquid CO.sub.2 29 recovered at the bottom.

[0088] In order to provide the refrigeration necessary for the correct operation of the CO.sub.2 purification/liquefaction unit 28, the refrigerant fluid (i.e. here the liquid stream 18″) circulates between the CO.sub.2 purification/liquefaction unit 28 and the natural gas liquefaction unit. Thus, owing to this thermal integration, a refrigeration cycle dedicated to the CO.sub.2 purification/liquefaction unit is avoided.

[0089] A stream 29 comprising more than 95 mol % of liquefied CO.sub.2, preferably more than 99 mol %, is extracted from this unit 28 and will be able to be used for various applications, for example in the food sector. Preferably, the stream 29 contains more than 99.5 mol % of CO.sub.2.

[0090] 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.