Method and apparatus for separating air by cryogenic distillation

Abstract

A method for separating air is provided, in which a flow of oxygen-rich liquid is sent to a top of a pure oxygen column, having a pure oxygen reboiler, in which said flow is purified in order to form a vessel liquid containing at least 98 mol % of oxygen and the vessel liquid is drawn off as a product. A supercharged airflow at a second pressure is sent to the pure oxygen reboiler and to a liquid oxygen vaporizer; a nitrogen-rich gas is drawn from the top of the medium-pressure column and sent to an intermediate reboiler of the low-pressure column and the condensed gas is sent to the top of the medium-pressure column; and a nitrogen-rich gas or air is sent to a vessel reboiler of the low-pressure column and the liquid that condenses therein is sent to the medium-pressure column.

Claims

1. A method for separating air by cryogenic distillation in a separation unit comprising a medium-pressure column and a low-pressure column, connected thermally together, with the low-pressure column comprising a vessel reboiler and an intermediate reboiler, and a pure oxygen column, the method comprising the steps of: i) introducing a purified and cooled gaseous air at a first pressure from an exchange line to the medium-pressure column; ii) sending an oxygen-rich liquid and a nitrogen-rich liquid from the medium-pressure column to the low-pressure column; iii) withdrawing a nitrogen-rich gas from the low-pressure column; iv) withdrawing an oxygen-rich liquid containing at most 97 mol % oxygen from the vessel reboiler of the low-pressure column; v) sending a first flow of oxygen-rich liquid to a vaporizer and sending the gaseous oxygen formed to the exchange line; vi) sending a second flow of oxygen-rich liquid to the top of the pure oxygen column, the pure oxygen column having a pure oxygen reboiler, wherein the second flow of oxygen-rich liquid is purified in order to form a vessel liquid containing at least 98 mol % oxygen; vii) sending a boosted airflow at a second pressure, higher than the first pressure, to the vessel reboiler of the pure oxygen column; viii) withdrawing a nitrogen-rich gas from the top of the medium-pressure column and sending the nitrogen-rich gas to the intermediate reboiler of the low-pressure column, and sending the condensed gas to the top of the medium-pressure column; and ix) sending a nitrogen-rich gas or air to the vessel reboiler of the low-pressure column and sending the liquid that condenses therein to the medium-pressure column, x) withdrawing an oxygen-rich top gas from the pure oxygen column and introducing said oxygen-rich top gas to the low-pressure column or to the exchange line for warming, wherein the vessel liquid is withdrawn from the pure oxygen column as a product and boosted air at the second pressure is sent to the vaporizer in order to vaporize the first flow of oxygen-rich liquid, wherein the boosted air at the second pressure is divided into two portions, a first portion of boosted air at the second pressure is sent to the vessel reboiler of the pure oxygen column and a second portion of boosted air at the second pressure is sent to the vaporizer, wherein the first flow of oxygen-rich liquid is partially vaporized in the vaporizer, with the liquid formed constituting the second flow of oxygen-rich liquid, wherein the first flow of oxygen-rich liquid is less rich in oxygen than the second flow of oxygen-rich liquid, wherein the first flow of oxygen-rich liquid is pressurised upstream of the vaporizer.

2. The method as claimed in claim 1, wherein the boosted air flow is at a pressure of about 4.5 bara.

3. The method as claimed in claim 1, wherein the bubble point of the boosted air flow has a bubble point higher than an equilibrium temperature of the vessel liquid from the pure oxygen column.

4. The method as claimed in claim 1, wherein the boosted air flow is at a temperature that is 2 C. to 3 C. warmer than the second flow of oxygen-rich liquid in the pure oxygen reboiler.

5. The method as claimed in claim 1, wherein the pure oxygen column is at a pressure that is equal to the pressure of the low pressure column.

6. The method as claimed in claim 1, wherein a cryogenic liquid from an auxiliary source is sent to the double column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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.

(2) FIG. 1 shows an embodiment of the invention.

(3) FIG. 2 shows an embodiment of the invention.

DETAILED DESCRIPTION

(4) The invention shall be described in more detail by referring to the figures, which show methods for separating air according to the invention.

(5) In FIG. 1, the air is separated in an ASU comprising a double column for separating air, comprising a medium-pressure column 23 and a low-pressure column 25. Frigories for the separation are provided via the expansion of medium-pressure nitrogen in a turbine 47. The apparatus comprises a column of pure liquid oxygen 49, a pump 57, a vaporiser 51 and an exchange line 63.

(6) The air 1 is pressurised by a compressor 3 at a pressure between 2.5 and 4.5 bar abs. The air is then purified in a purification unit 5 via adsorption. The purified air 7 is divided into two portions. One portion 9 is boosted in a booster 13 to a pressure between 4 and 20 bar abs and is then cooled in the exchange line 63 until cold. The air 9 is divided into two fractions 15, 17. One fraction 15 is sent to the vaporiser 51 where it is used to partially vaporise liquid oxygen comprising at most 97 mol % oxygen, in order to produce gaseous oxygen 59 which is heated in the exchange line 63. This gas 59 is sent to an oxycombustion unit. An oxygen-rich liquid 53 is withdrawn from the vaporiser 51 as a purge. The air is condensed. The other fraction of the air 17 is sent to the vessel reboiler 61 of the pure oxygen column 49. This column comprises the vessel reboiler and means for exchanging heat and material above this reboiler. Liquid oxygen 65 comprising at most 97 mol % oxygen is sent to the top of the column 49 and is enriched in order to form the liquid product 71 withdrawn from the vessel and containing at least 98 mol % oxygen. The gaseous oxygen from the top of the column 49 is sent to the vessel of the low-pressure column 25. The condensed air 17 is mixed with the condensed air coming from the vaporiser 51 and, after expansion in a valve 21, is sent to the MP column 23, which operates at between 2.5 and 4.5 bar abs.

(7) Another portion 11 of the air is cooled in the exchange line 63, is sent to the vessel reboiler 35 of the LP column 25, is condensed therein at least partially and is sent to the vessel of the MP column 23, below the inlet of liquid air 19.

(8) Oxygen-rich liquid 27 is withdrawn from the vessel of the MP column 23, cooled in the sub-cooler 33, expanded and sent to the LP column 25. Liquid 29 is withdrawn from the MP column 23, cooled in the sub-cooler 33, expanded and sent to the LP column 25. Nitrogen-rich liquid 31 is withdrawn from the top of the MP column 23, cooled in the sub-cooler 33, expanded and sent to the top of the LP column 25.

(9) Low-pressure nitrogen 39 is withdrawn from the top of the LP column, heated in the sub-cooler 33 and heated in the exchange line 63.

(10) Medium-pressure nitrogen 41 is divided into two in order to form a portion 43 and a portion 45. The portion 43 is used to heat the intermediate reboiler 37 of the low-pressure column 25. The portion 45 is heated in the exchange line 63, is expanded in the turbine 47 and is sent back to the exchange line 63. Liquid oxygen is withdrawn from the vessel of the LP column and divided into two. A portion 55 is pressurised in the pump 57 upstream of the vaporiser 51 and the rest 65 is sent to the top of the pure oxygen column 49 without having been pressurised. The top of the pure oxygen column 49 is therefore at the same pressure as the vessel of the low-pressure column 25. All or a portion of the purge liquid 53 can also supply the top of the column 49.

(11) A flow of cryogenic liquid 69, for example liquid nitrogen, is sent to the top of the LP column in order to keep the method cooled.

(12) The method in FIG. 1 a differs from that of FIG. 1 in that the column 49 is supplied at the top exclusively by the purge 53 of the vaporiser 51, following an expansion step in a valve. The vessel reboiler 61 of the column 49 is still heated by the boosted air 17, with the air condensed being mixed with the boosted air 15 which was used to heat the vaporiser 51. It is also possible to supply the column with purge liquid 53 and liquid oxygen 65 coming from the vessel of the low-pressure column 25.

(13) The method of FIG. 2 differs from that of FIG. 1 in that the airflow 9 is first sent to the vessel vaporiser 61 of the pure oxygen column 49 and then to the vaporiser 51 where it is condensed. The air formed is expanded in the valve 21 and sent to the medium-pressure column 23. The fraction of air 11 is cooled in the exchange line 11 and is sent to the vessel of the medium-pressure column 23 without having been expanded or compressed downstream of the compressor 3.

(14) The intermediate reboiler 37 is always heated by medium-pressure nitrogen 43 but another portion of the medium-pressure nitrogen 73 is compressed in a cold booster 71 using a cryogenic temperature and sent to the vessel reboiler 35. The condensed nitrogen is expanded in a valve 36 and sent to the top of the MP column 23. The vessel oxygen 55 of the low-pressure column is entirely pressurised in the pump 57 sent to the vaporiser 51 where it is partially vaporised. The vaporised gas constitutes the gaseous oxygen product 59 containing less than 97 mol % oxygen. The non-vaporised liquid 53 supplies the top of the column 49. The gaseous oxygen 67 from the top of the column 49 is mixed with the gaseous oxygen 59. The liquid oxygen 71 constitutes the liquid product. In this case, the pure oxygen column 49 does not operate at the same pressure as the LP column 25.

(15) The method in FIG. 1 or 1 a can use nitrogen to heat the vessel reboiler 35 and the method in FIG. 2 can use air to heat the vessel reboiler 35.

(16) 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.

(17) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(18) 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.

(19) 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.

(20) 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.

(21) 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.

(22) 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.