METHOD FOR LIQUEFYING A STREAM RICH IN CO2

Abstract

In a method for liquefying a gas rich in carbon dioxide, the gas is compressed to a first pressure greater than its critical pressure in a compressor to form a compressed gas, the compressed gas is cooled through heat exchange with a refrigerant to a variable temperature to form a cooled compressed gas with a density between 370 and 900 kg/m.sup.3, the cooled compressed gas is cooled at supercritical pressure in a first heat exchanger to a temperature below the critical temperature, the gas cooled below the critical temperature is expanded to a second pressure between 45 and 60 bara to form a diphasic fluid which is separated in a phase separator to form a liquid and a gas, and a liquid portion originating from the phase separator provides cold to the first heat exchanger.

Claims

1-14. (canceled)

15. A process for the liquefaction of a gas rich in carbon dioxide, in which: i. compressing the gas to a first pressure greater than its critical pressure in a compressor to form a compressed gas; ii. cooling the compressed gas by heat exchange with a refrigerant having a variable temperature to form a cooled compressed gas with a density between 370 and 900 kg/m.sup.3, indeed even between 450 and 750 kg/m.sup.3; iii. cooling the cooled compressed gas at the supercritical pressure in a first heat exchanger down to a temperature below the critical temperature, for example between 5? C. and 25? C.; iv. expanding the gas cooled below the critical temperature to a second pressure between 45 and 60 bara to form a liquid or a two-phase fluid which is optionally separated in a phase separator to form a liquid and a gas; wherein a part of the liquid or of the two-phase fluid from step iv) or of the liquid originating from the phase separator supplies cold to the first heat exchanger; and wherein a part of the liquid or of the two-phase fluid from step iv) or of the liquid originating from the phase separator is supercooled to form a supercooled liquid: wherein, if the temperature of the refrigerant of step ii) lessens, the first pressure is reduced and/or, if the temperature of the refrigerant of step ii) increases, the first pressure is increased, and/or if the temperature of the refrigerant of step ii) lessens, the flow of liquid or of two-phase fluid sent to the first heat exchanger is reduced and/or, if the temperature of the refrigerant, for example air or water, of step ii) increases, the flow of liquid or of two-phase fluid sent to the first heat exchanger is increased.

16. The process as claimed in claim 15, wherein the part of the liquid or of the two-phase fluid which provides cold in the first heat exchanger heats up in the first heat exchanger and preferably vaporizes therein.

17. The process as claimed in claim 15, wherein the part of the liquid or of the two-phase fluid which supplies cold to the first heat exchanger is sent after vaporization to be mixed with the gas to be compressed.

18. The process as claimed in claim 15, wherein the part of the liquid or of the two-phase fluid from step iv) or the part of the liquid originating from the phase separator is supercooled in a second heat exchanger and sent to be separated in a distillation column and a liquid enriched in carbon dioxide is withdrawn from the distillation column as product.

19. The process as claimed in claim 18, wherein a part of the bottom liquid from the distillation column vaporizes in the second heat exchanger and is sent to the compressor in order to be compressed therein with the gas to be separated.

20. The process as claimed in claim 19, wherein the compressor comprises at least two stages and the vaporized bottom liquid is sent to the compressor at a lower pressure than that at which the liquid vaporized in the first heat exchanger is sent thereto.

21. The process as claimed in claim 15, wherein at least a part of the gas formed in the phase separator is discharged to the atmosphere.

22. The process as claimed in claim 21, wherein, if, preferably only if, the gas to be liquefied becomes less rich in carbon dioxide, the flow of gas formed in the phase separator discharged to the atmosphere is increased.

23. The process as claimed in claim 15, wherein at least a part of the gas formed in the phase separator is cooled in a second heat exchanger before being sent to a distillation column.

24. The process as claimed in claim 15, wherein at least a part of the gas formed in the phase separator is sent to the compressor in order to be compressed therein with the gas to be separated.

25. The process as claimed in claim 15, wherein the gas to be separated is compressed in the at least two stages of the compressor.

26. The process as claimed in claim 15, wherein h the density of the liquid from step iv) or of the two-phase fluid is between 750 and 950 kg/m.sup.3, indeed even between 820 and 850 kg/m.sup.3.

27. The process as claimed in claim 15, wherein the first pressure is regulated in order to have one of N pressures, N being an integer between 2 and 5, whatever the temperature of the refrigerant.

28. The process as claimed in claim 15, wherein, during a first period, the refrigerant is at an average temperature of T? C. and, during a second period, the refrigerant is at an average temperature of T+20? C. and in which the density of the liquid from step iv) or of the liquid fraction of the two-phase fluid during the first period differs from that during the second period only by at most 100 kg/m.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments

[0040] FIG. 1 illustrates a process according to the invention.

[0041] FIG. 2 is a Mollier chart showing several cooling steps according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0042] A gas 1 containing at least 80 mol %, preferably at least 95 mol %, of carbon dioxide as well as impurities, such as carbon monoxide, nitrogen or oxygen, is compressed in the first part 10 of a compressor, comprising at least one stage. Subsequently, it is compressed in the second part 20 of the compressor, comprising at least one stage, to reach a pressure greater than the critical pressure. The supercritical fluid 2 is cooled in a cooler 30 by a refrigerant, the temperature of which is liable to change with the ambient temperature, for example air or water, to form a supercritical fluid 3 with a density between 370 and 900 kg/m.sup.3. The gas 3 is cooled in a heat exchanger 40 to reach a subcritical temperature, for example between 5? C. and 25? C. The fluid 4 formed is expanded in a valve 50 to a pressure between 45 and 60 bara to form a two-phase fluid which is subsequently separated in a phase separator 60.

[0043] A part 8 of the liquid from the phase separator 60 is used to cool the first heat exchanger 40. In this example, the liquid 8 vaporizes in the exchanger 40 and is returned to the gas to be separated 1 between the two parts 10, 20 of the compressor.

[0044] In addition, the transfer of cold to the heat exchanger can be carried out through another means; thus, the liquid 8 is not necessarily sent itself into the first exchanger 40.

[0045] It is also possible for the expansion in the valve 50 to be able to produce a single-phase liquid. In this case, the phase separator 60 is not necessary. A part of this liquid will thus be used to supply cold to the first exchanger 40.

[0046] Certain usages of the liquid expanded in the valve 50 or of types of exchangers for supercooling it tolerate the presence of a certain amount of gas in the liquid. In these cases also, the presence of the phase separator 60 is optional.

[0047] The liquid produced after the expansion in the valve 50 can constitute the product of the process, without subsequent purification in a column or the like. It is the same for the liquid 5 from the phase separator.

[0048] Alternatively, as illustrated in the figure, the liquid 5 (or the liquid expanded in the valve 50 if the separator 60 is not present) is divided into two parts. The part 8, which has already been mentioned, serves to cool the first exchanger and the part 9 is cooled in the heat exchanger 70, which is a brazed aluminum plate heat exchanger.

[0049] In this example, the liquid 9 is cooled down to an intermediate temperature, then divided into two to form two flows 11, 12. The first flow 11 leaves the exchanger 70 at an intermediate temperature, is expanded in a valve 110 and then sent to the distillation column 80 at an intermediate level thereof. The second flow 12 is cooled as far as the cold end of the exchanger 70, is expanded in a valve 100 and is sent to the top of the column 80. It is also possible to envisage other ways of feeding the column, for example by sending a single liquid flow to the column. The column can also be replaced by at least one phase separator.

[0050] The bottom liquid from the column 80 enriched in carbon dioxide is divided into three parts. A part 13 is heated in the exchanger 70 and sent to the bottom of the column 80 to contribute heat. A part 19 is supercooled in the exchanger 80 to form a liquid product of the process.

[0051] A part 17 is expanded in a valve 120 and vaporizes in the heat exchanger 70 to form a gas which is mixed with the gas 1 and is compressed in the compressor 10, 20.

[0052] The gas 7 from the phase separator 60, if present, can be sent to the air and/or cooled in the heat exchanger 70 as gas 10, expanded and sent to feed the column 80 and/or sent to the compressor 10, 20 to be compressed with the gas 1.

[0053] The cooler 30 is cooled by a refrigerant having a variable temperature, for example air or water. According to the temperature of the refrigerant, the density of the gas 3 cooled in the cooler can vary between 370 and 900 kg/m.sup.3. Due to the use of the first heat exchanger, the density of the liquid obtained after the valve 50 remains between 750 and 950 kg/m.sup.3, indeed even between 820 and 850 kg/m.sup.3, whatever the temperature of the refrigerant. Thus, the variation in density is greatly alleviated for the liquid produced.

[0054] If the temperature of the refrigerant increases (for example if the ambient temperature increases in the case of water and air), the compressor 10, 20 produces a flow 2 at a higher pressure in order to achieve the desired density.

[0055] If the temperature of the refrigerant lessens (for example if the ambient temperature lessens in the case of water and air), the compressor 10, 20 produces a flow 2 at a lower pressure in order to achieve the same desired density. In this case, the amount of cold sent to the first heat exchanger 40 will be reduced, for example by reducing the flow of gas 8 sent to the heat exchanger 40 in order to cool it. This makes it possible to obtain an unvarying density downstream of the valve 50.

[0056] Preferably, the heat exchanger 40 makes possible the direct exchange of heat between the flow 3 and the flow 8 only.

[0057] If the composition of the flow 1 varies and becomes less rich in carbon dioxide, the phase separator 60 sends more gas 7 to the atmosphere in order for the liquid product 5 from the phase separator 60 to have a virtually unvarying composition. This makes it possible for the heat exchanger 70 (if present) to cool a liquid having a stabilized composition.

[0058] The Mollier table of [FIG. 2] makes it possible to illustrate the cooling of a flow of pure CO.sub.2. If the refrigerant, for example cooling water, is at 35? C., the CO.sub.2 is at 40? C. (counting 5? C. of approach) and if a density of 600 kg/m 3 is regarded as sufficient at the inlet of the exchanger 40, there must be a pressure of more than 90 bara (point 1). On the other hand, if the water is colder, 30? C. for example, the CO.sub.2 is at 35? C. and, with the same density, only 80 bara is needed (point 2). Finally, if the water is at 15? C., the CO.sub.2 at 20? C., it will be liquefied in the vicinity of 60 bara (point 3).

[0059] In operation, the compressor 10, 20 will be regulated with an outlet pressure value which is a function of the temperature of the CO.sub.2 obtained after cooling by the refrigerant. In order to avoid excessively large and excessively frequent fluctuations (for example between day and night), pressure set points can be defined in advance for several weeks/months. With the example above, it might be possible to imagine operating in summer at more than 90 bara, in winter at approximately 60 bara and the rest of the time at 80 bara. The inlet guide vanes of the compressor 10, 20 will be adjusted in order to modify the outlet pressure.

[0060] If the process is continued, cooling will be carried out down to a temperature of 20? C. according to the preceding example (points 4 and 5), then expanding will be carried out to a pressure lower than the critical pressure, 50 bara in the example (points 7, 8, 9). The formation is then observed of a liquid/vapor mixture and thus, whatever the starting point (among the points 1, 2, 3), points which are very close and at the same density are obtained. The liquid can thus go into the cryogenic part with the same conditions, whatever the temperature of the refrigerant (point 6). The only difference is the gas/liquid fraction obtained but the points 7, 8, 9 are very close, the fraction then being not greatly impacted.

[0061] The example of the Mollier table above is for pure CO.sub.2. With light impurities, it would be necessary to rise higher in pressure in order to reach the same density. After cooling and expansion, the vapor fraction generated (point 10 in the case of pure CO.sub.2) is concentrated in light impurities. Furthermore, the more light impurities there are, the more the vapor fraction increases and thus its flow. As the vapor flow is increasing, the pressure within the pot 60 will increase. It will be possible to decide to open a valve toward venting 7 when the value reached will be excessively high.

[0062] Downstream of the valve, the density at 45 bara is 870 kg/m 3 and 765 kg/m.sup.3 at 60 bara if pure CO.sub.2 was concerned. With a small amount of light products, this density will vary downward.

[0063] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0064] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0065] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

[0066] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0067] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0068] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0069] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.