Method for Separating a Carbon Dioxide-rich Gas by Partial Condensation and Permeation

Abstract

A carbon dioxide-rich gas is cooled in a first heat exchanger; the carbon dioxide-rich gas cooled in the first heat exchanger enters a first phase separator at a first temperature between −50° C. and −53° C.; a carbon dioxide-enriched liquid is drawn from the first phase separator and acts as product; a carbon dioxide-depleted gas from the first separator is reheated in the first exchanger to a second temperature between −35° C. and −45° C., without having been expanded downstream from the first phase separator and enters a permeation unit at the second temperature; the reheated gas is subjected to at least one permeation step in the permeation unit in order to produce at least two fluids which are then reheated in the first exchanger, the fluid from the permeation unit, reheated in the exchanger, which exits the permeation unit at the lowest temperature, being at a temperature of more than −54° C.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for separating a gas rich in carbon dioxide by partial condensation and permeation.

BACKGROUND

A gas rich in carbon dioxide contains at least 65 mol %, or at least 80 mol % or even at least 90 mol % of carbon dioxide, on a dry basis.

The gas rich in carbon dioxide contains at least one lighter gas, such as oxygen, nitrogen, carbon monoxide, hydrogen.

Preferably, it contains less than 5 mol % of methane, or even less than 2 mol % of methane.

All the percentages in this document relating to purities are molar percentages.

The low temperature separation of a gas rich in carbon dioxide is particularly chosen as a means of capturing CO₂ when the concentration of the gas is high enough (≧30 mol %). Before being cooled (usually as close as possible to the triple point of CO₂), generally the gas must be dried and its pressure must be increased for the purpose of capturing the CO₂ at the desired yield, since the separation is essentially based on partial condensation.

It is possible to increase this yield by means of membranes placed on the top of the partial condensation pot as illustrated in WO-A-2012/048078: specifically, the pressure of the non-condensable gases that will be recycled in the membranes is taken advantage of.

Membranes will be chosen in which the CO₂ will permeate preferably with respect to the other compounds in order to capture the CO₂ in the permeate from the membranes.

Membranes capable of carrying out such a separation are known from WO-A-2011/084516 and from “CO₂ Capture by Subambient Membrane Operation” by Kulkarni et al., 2010 NETL CO₂ Capture Technology.

Details of membranes suitable for the present invention and the operation thereof are given in these documents.

The loss of pressure between the inlet gas of the membranes and the permeate involves a large temperature drop. This temperature drop will be applied both to the permeate and to the residue resulting from the membranes since the permeate will exchange heat with the residue.

The membrane thus acts as a heat exchanger, to the extent that in certain configurations (such as counter flow) the temperature drop is greater for the residue than for the permeate.

Furthermore, since the gas resulting from the condensation pot is at phase equilibrium and close to the triple point, an excessively large temperature drop could liquefy CO₂, or even freeze it in the membrane, which could degrade its performances or even threaten its integrity.

Finally, if the temperature of the residue and/or of the permeate drops below the temperature of the triple point (which will easily be achieved for high membrane yields), recycling the cold from these fluids in an exchanger will prove complex. Specifically, since these fluids are particularly cold, it will be sought to cool CO₂ against them. The skin temperature at the exchange (tubes or plates for example) would then be very low and the risks of solidification of the CO₂ to be cooled would be very high.

SUMMARY OF THE INVENTION

The present invention relates to the adaptation of the outlet temperature of the membranes in order to prevent excessively low temperatures involving risks of solidification of the CO₂ during the recycling of the cold from the fluids at the outlet of the membranes.

The invention therefore consists in limiting the temperature drop to a temperature above the triple point of the CO₂ in order to be able to ensure the recycling of the cold generated by the membrane separation in the remainder of the process.

In order to limit the temperature drop, it is possible to increase the operating temperature of the membranes by warming the top of the pot.

Several options are then possible:

• • warming and recycling the cold from the fluids resulting from the membrane in the main exchange line (FIG. 1);
  • warming in a dedicated exchanger (FIG. 2), in particular when the exchange line is separated into two parts with the second (coldest) part using a technology that prevents the cold box (shell and tubes for example) from freezing solid.

In this diagram, the LP CO₂ is subcooled, before the expansion thereof, against all the gases whose temperature is significantly lower than that of the exchange line (that of the higher temperatures). This makes it possible, in this way, to recycle the low-temperature cold against hotter fluids at similar temperatures which optimizes the system by minimizing exergetic losses during heat exchanges;

• • warming the top of the phase separator and intermediate warming (FIG. 3). The performances of the cryogenic membranes are more advantageous when the temperature is minimized. In order to optimize the membrane separation, the top of the pot will therefore be warmed less than in the preceding diagrams and in order to limit the temperature drop re-warming will be carried out after emerging from a first batch of modules and before going into the last membrane modules. These two warming operations may be carried out as above in the main exchange line or in a dedicated exchanger.

It will be possible to repeat the intermediate warming operations as many times as possible in order to optimize the system.

Irrespective of the chosen solution, care will be taken to adapt the pressure drop in the exchangers in the passes of the permeate(s) of the membranes. Specifically, since the pressure on the permeate side is particularly low, the volume flow rate of the gas is very high, which greatly impacts the size of the exchangers. Choosing high pressure drops would make it possible to decrease the flow area of the permeate(s) thus decreasing the size of the exchanger.

According to one subject of the invention, a process is provided for separating a gas rich in carbon dioxide by partial condensation and permeation, wherein the gas rich in carbon dioxide is cooled at least in a first heat exchanger, the gas rich in carbon dioxide cooled in the first heat exchanger or a fluid derived from this gas goes into a first phase separator at a first temperature between −50° C. and −53° C., a liquid enriched in carbon dioxide is withdrawn from the first phase separator and serves as product or is treated in order to enrich it even more in carbon dioxide, a gas depleted in carbon dioxide from the first phase separator is warmed in the first exchanger or a second exchanger to a second temperature between −35° C. and −45° C., without having been expanded downstream of the first phase separator and goes into a permeation unit at the second temperature, the warmed gas undergoes at least one permeation step in the permeation unit in order to produce at least two fluids that are then warmed in the exchanger in which the depleted gas was warmed, the fluid resulting from the permeation unit, warmed in the exchanger, which leaves the permeation unit at the lowest temperature being at a temperature above −54° C.

According to other optional aspects:

• • the depleted gas is warmed in the second exchanger and the at least two fluids resulting from the permeation being warmed in the second exchanger, the second exchanger not being used to cool the gas rich in carbon dioxide;
  • a fluid originating from the treatment of the liquid enriched in carbon dioxide is sent to the second exchanger at a temperature below −40° C., in order to warm the depleted gas and the at least two fluids resulting from the permeation;
  • the fluid originating from the liquid treatment is a liquid that contains at least 90% carbon dioxide produced by distillation and expanded in a valve;
  • the depleted gas is warmed in the first exchanger just like the fluids resulting from the permeation;
  • the permeation unit makes it possible to carry out a single permeation step and the permeate and the residue of the permeation step are sent to be warmed in the first or second heat exchanger;
  • the permeation unit makes it possible to carry out a first permeation step, producing a permeate and a residue, at least one portion of the residue then being separated by a second permeation step with an inlet temperature between −40° C. and −45° C., at least two of the fluids chosen from the list at least one portion of a residue from the first step and/or at least one portion of a residue from the second step and/or at least one portion of a permeate from the second step and/or at least one portion of a permeate from the first step being warmed in the first or second exchanger;
  • the gas rich in carbon dioxide is sent from the first heat exchanger to the first phase separator without having been separated by permeation;
  • the gas rich in carbon dioxide is treated by a step of partial condensation and the gas formed is at least partially condensed in order to form the fluid derived from the gas rich in carbon dioxide which goes into the first phase separator.

According to another aspect of the invention, an apparatus is provided for separating a gas rich in carbon dioxide by partial condensation and permeation, comprising a first heat exchanger, means for sending the gas rich in carbon dioxide to be cooled at least in a first heat exchanger, a first phase separator, means for sending the gas rich in carbon dioxide cooled in the first heat exchanger or a fluid derived from this gas to the first phase separator at a first temperature between −50° C. and −53° C., means for withdrawing a liquid enriched in carbon dioxide from the first phase separator in order to serve as product or in order to be treated to enrich it even more in carbon dioxide, means for sending a gas depleted in carbon dioxide from the first phase separator to be warmed to an intermediate temperature of the first exchanger or of a second exchanger, the intermediate temperature being a second temperature between −35° C. and −45° C., without means for expanding the gas depleted in carbon dioxide downstream of the first phase separator, means for sending the warmed depleted gas to a permeation unit at the second temperature, means for extracting at least two fluids from the permeation unit, means for sending the at least two fluids to be warmed in the exchanger in which the depleted gas was warmed so that the fluid resulting from the permeation unit and warmed in the exchanger, which leaves the permeation unit at the lowest temperature, is at a temperature above −54° C.

BRIEF DESCRIPTION OF THE DRAWINGS

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.

FIG. 1 provides an embodiment of the present invention.

FIG. 2 provides an embodiment of the present invention.

FIG. 3 provides an embodiment of the present invention.

DETAILED DESCRIPTION

The invention will be described in greater detail by referring to the figures. FIGS. 1 to 3 represent processes according to the invention.

In FIG. 1, an oxy-fuel combustion unit 1 produces a gas stream 3 rich in carbon dioxide that contains at least 80% carbon dioxide on a dry basis, and also at least oxygen, nitrogen and argon. The gas is cooled in a first heat exchanger 5 which is a brazed aluminum plate-fin exchanger. The cooled gas 7 at a temperature between −50° C. and −53° C. is sent to a phase separator 9 which produces a gas 11 enriched in oxygen, nitrogen and argon that contains 34% carbon dioxide and a liquid 13 enriched in carbon dioxide. The liquid 13 is sent to a storage vessel 15 as final product. The gas 11 is warmed in the first exchanger 5 to between −35° C. and −45° C., for example −40° C. and then sent to a permeation separation stage 17. This permeation causes a cooling of the products. The permeate 19 at −42° C. is expanded and then sent to be warmed in the first exchanger 5. The residue 21 at −51° C. is also sent to be warmed in the first exchanger 5. The coldest of the two fluids 19, 21 is at a temperature above −54° C. at the outlet of the unit 17.

In FIG. 2, an oxy-fuel combustion unit 1 produces a gas stream 3 rich in carbon dioxide that contains at least 65% carbon dioxide, and also at least oxygen, nitrogen and argon. The gas is cooled in a first heat exchanger 5 which is a brazed aluminum plate-fin exchanger. The cooled gas 7 is sent to a phase separator 23 in order to produce a gas 25 enriched in oxygen, nitrogen and argon and a liquid 27. The liquid 27 is expanded and supplied to the top of a low-temperature separation column 15. The gas 25 is partially condensed in a shell and tube exchanger 29 in order to produce a diphasic stream 31. This stream 31 is at a temperature between −50° C. and −53° C. and is sent to a phase separator 9 which produces a gas 11 enriched in oxygen, nitrogen and argon and a liquid 13 enriched in carbon dioxide. The liquid 13 is sent to the top of the column 15. The gas 11 is warmed in a second exchanger 105 to between −35° C. and −45° C. and is then sent to a permeation separation stage 17. This permeation causes a cooling of the products. The permeate 19 is expanded and then sent to be warmed in the second exchanger 105. The residue 21 is also sent to be warmed in the second exchanger 105. The coldest of the two fluids 19, 21 is at a temperature above −54° C. at the outlet of the unit 17. The bottoms liquid 33 from the distillation column is cooled in the second exchanger 105. A cold fluid 47 at less than −40° C. may be warmed in the exchanger 105. A fluid 49 is cooled therein. The liquid 33 subcooled in the second exchanger 105 is vaporized in the exchanger 29 in order to form a gaseous product that is then warmed in the first exchanger 5.

In FIG. 3, an oxy-fuel combustion unit 1 produces a gas stream 3 rich in carbon dioxide that contains at least 80% carbon dioxide, and also at least oxygen, nitrogen and argon. The gas is cooled in a first heat exchanger 5 which is a brazed aluminum plate-fin exchanger. The cooled gas 7 at a temperature between −50° C. and −53° C. is sent to a phase separator 9 which produces a gas 11 enriched in oxygen, nitrogen and argon and that contains 34% carbon dioxide and a liquid 13 enriched in carbon dioxide. The liquid 13 is sent to a storage vessel 15 as final product. The gas 11 is warmed in the first exchanger 5 to between −35° C. and −45° C., for example −45° C. and then sent to two permeation separation stages in series 17, 117 forming a permeation unit. This permeation causes a cooling of the products. The permeate 19 from the first permeation stage 17 at −50° C. is sent to be warmed in the first exchanger 5. The residue 21 at −53° C. is also sent to be warmed in the first exchanger 5 to an intermediate temperature thereof (−45° C.) and then supplies the second permeation stage 117 in order to produce a permeate 41 at −47° C. and a residue 43 at −50° C. The residue is warmed in the first exchanger and the permeate 41 is mixed with the permeate 19 in order to make a stream 45 at −49° C. The coldest of the four fluids 19, 21, 41, 43 which is the residue 21 from the first stage is at a temperature above −54° C. at the outlet of the unit 17, 117.

As the yield from the first stage is 64% and that from the second is 79%, the overall yield of the permeation unit is 92%.

It is also possible to implement the invention in the variant from FIG. 3 without warming the residue 21 from the first stage 17. This version is not illustrated. In this case, the gas 11 is at −45° C., the permeate 19 is at −42° C. and the residue 21 is at −51° C. The residue arrives in the second stage 117 still at −51° C.

The permeate produced 41 is at −54° C. and the residue 43 is at −54° C. The two permeates are mixed in order to form a stream at −45° C. As the yield from the first stage is 75% and that from the second is 78%, the overall yield of the permeation unit is 92%.

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.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. “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.

“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 a range is expressed, it is to be understood that another embodiment is from the one.

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.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

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.

Claims

1-9. (canceled)

10. A process for separating a gas rich in carbon dioxide by partial condensation and permeation, the process comprising the steps of: cooling the gas rich in carbon dioxide at least in a first heat exchanger;sending the gas rich in carbon dioxide cooled in the first heat exchanger or a fluid derived from this gas into a first phase separator at a first temperature between −50° C. and −53° C.;withdrawing a liquid enriched in carbon dioxide from the first phase separator to be used as product or further treating the liquid enriched in carbon dioxide in order to further enrich the liquid enriched in carbon dioxide;withdrawing a gas depleted in carbon dioxide from the first phase separator and warming the gas in the first exchanger or a second exchanger to a second temperature between −35° C. and −45° C., without having been expanded downstream of the first phase separator; andsending the gas depleted in carbon dioxide to a permeation unit at the second temperature, wherein the warmed gas undergoes at least one permeation step in the permeation unit in order to produce at least two fluids that are then warmed in the exchanger in which the depleted gas was warmed, the fluid resulting from the permeation unit, warmed in the exchanger, which leaves the permeation unit at the lowest temperature being at a temperature above −54° C.

11. The process as claimed in claim 10, wherein the depleted gas is warmed in the second exchanger and the at least two fluids resulting from the permeation being warmed in the second exchanger, the second exchanger not being used to cool the gas rich in carbon dioxide.

12. The process as claimed in claim 11, wherein a fluid originating from the treatment of the liquid enriched in carbon dioxide is sent to the second exchanger at a temperature below −40° C., in order to warm the depleted gas and the at least two fluids resulting from the permeation.

13. The process as claimed in claim 12, wherein the fluid originating from the liquid treatment is a liquid that contains at least 90% carbon dioxide produced by distillation and expanded in a valve.

14. The process as claimed in claim 10, wherein the depleted gas is warmed in the first exchanger just like the fluids resulting from the permeation.

15. The process as claimed in claim 10, wherein the permeation unit makes it possible to carry out a single permeation step and the permeate and the residue of the permeation step are sent to be warmed in the first or second heat exchanger.

16. The process as claimed in claim 10, wherein the permeation unit makes it possible to carry out a first permeation step, producing a permeate and a residue, at least one portion of the residue then being separated by a second permeation step with an inlet temperature between −40° C. and −45° C., at least two of the fluids chosen from the list at least one portion of a residue from the first step and/or at least one portion of a residue from the second step and/or at least one portion of a permeate from the second step and/or a portion of a permeate from the first step being warmed in the first or second exchanger.

17. The process as claimed in claim 10, wherein the gas rich in carbon dioxide is sent from the first heat exchanger to the first phase separator without having been separated by permeation.

18. The process as claimed in claim 10, wherein the gas rich in carbon dioxide is treated by a step of partial condensation and the gas formed is at least partially condensed in order to form the fluid derived from the gas rich in carbon dioxide which goes into the first phase separator.