Device and method for separating air by cryogenic distillation

Abstract

Method for separating air by cryogenic distillation, wherein at least part of the air to be distilled is boosted in an air booster, compressed air is allowed to expand in at least one expansion turbine and, if the pressure drop between two points of the booster passes under a threshold and/or a flow of the booster passes under a minimum flow of the booster, part of the air boosted in the booster is allowed to expand without having been cooled between the booster and the expansion turbine and the boosted expanded air is sent upstream or downstream of the at least one turbine, without having been cooled in the heat exchanger, after having been boosted.

Claims

1. A device for separating air by cryogenic distillation comprising: a main air compressor configured to compress all the air to be distilled; a cold booster configured to boost at least part of the air to be distilled; an expansion turbine configured to receive compressed air originating from the main air compressor; a system of cryogenic distillation columns comprising at least one column; a main heat exchanger in fluid communication with the main air compressor such that the main heat exchanger is configured to receive air from the main air compressor, the main heat exchanger having a warm end, a cold end, and an intermediate section located between the warm end and the cold end, wherein the intermediate section is in fluid communication with the cold booster such that the cold booster is configured to receive air from the intermediate section and then return boosted air to the main heat exchanger; wherein the expansion turbine is in fluid communication with the intermediate section and is configured to expand air from the intermediate section of the main heat exchanger and then send the expanded air to the system of columns, wherein the cold end of the main heat exchanger is in fluid communication with the system of columns, such that the cold end is configured to receive an oxygen enriched flow and a nitrogen enriched flow from the system of columns; an expansion valve in fluid communication with an outlet of the cold booster, the expansion valve being configured to expand air boosted in the cold booster to a lower pressure; an absence of a heat exchanger configured to cool air between the outlet of the cold booster and the expansion valve; wherein the device is further configured to determine that a pumping point is imminent upon a basis of finding: i) the pressure drop between two points of the cold booster is under a threshold, or ii) a flow of the cold booster is under a minimum flow of the cold booster, and then: expand a part of the air boosted in the cold booster without having been cooled between the cold booster and the expansion turbine, and then send the boosted expanded air upstream or downstream of the turbine, without having been cooled in the main heat exchanger therebetween; and in the event of case ii), the device is further configured to increase the flow in the cold booster in order to exit the pumping point.

2. The device according to claim 1, wherein the cold booster is connected to the inlet of the expansion turbine so that the boosted air can be allowed to at least partly expand in the expansion turbine.

3. A method for separating air by cryogenic distillation, the method comprising the steps of: compressing all air to be distilled in a main air compressor to form compressed air; cooling the compressed air in a main heat exchanger, the main heat exchanger having a warm end, a cold end, and an intermediate section located between the warm end and the cold end; introducing a first portion of cooled air from the intermediate section of the main heat exchanger to a cold booster to form a boosted air stream, and then sending the boosted air stream to the main heat exchanger for further cooling; introducing a second portion of cooled air from the intermediate section of the main heat exchanger to an expansion turbine, and then sending the expanded air to the system of columns; introducing a fully cooled air stream withdrawn from the cold end of the main heat exchanger to a system of cryogenic distillation columns comprising at least one column, wherein the system of cryogenic distillation columns are configured to produce an oxygen enriched stream and a nitrogen enriched stream; extracting the oxygen enriched stream and the nitrogen enriched stream from the system of columns and heating the oxygen enriched stream and the nitrogen enriched stream in the main heat exchanger; determining that a pumping point is imminent upon a basis of finding: i) the pressure drop between two points of the cold booster is under a threshold; or ii) a flow of the cold booster is under a minimum flow of the booster, and then in response to the pumping point being imminent, the method includes the steps of: expanding at least a fraction of the boosted air to form an expanded fraction of boosted air; sending the expanded fraction of boosted air to the system of columns for separation therein, without having been cooled in the main heat exchanger; and in the event of case ii), increasing the flow in the cold booster in order to exit the pumping point.

4. The method according to claim 3, further comprising the steps of determining that a pumping point is not imminent upon a basis of finding the pressure drop between the two points is above the threshold and/or a flow of the cold booster is above the minimum flow of the cold booster, and then sending all the air from the cold booster to the heat exchanger in order to be cooled.

5. The method according to claim 3, wherein, upon a determination that a pumping point is imminent, the method comprises an absence of recycling the boosted air to an inlet of the cold booster.

6. The method according to claim 3, wherein the expanded fraction of boosted air is further expanded in the expansion turbine prior to sending the expanded fraction of boosted air to the system of columns.

7. The method according to claim 3, wherein the expanded fraction of boosted air is expanded to the pressure of a column of the system of columns in an expansion valve, and then mixed with the air originating from the expansion turbine before being sent to the system of columns.

8. The method according to claim 3, further comprising the steps of determining that a pumping point is not imminent upon a basis of the pressure drop between the two points of the cold booster is above the threshold, and then sending all the boosted air to cool in the main heat exchanger.

9. The method according to claim 3, wherein the expansion turbine is coupled to the cold booster.

10. The method according to claim 3, wherein there is an absence of indirect contact cooling for the boosted air that is expanded and then sent to the system of columns between the outlet of the cold booster and the system of columns.

11. The method according to claim 3, wherein the expansion turbine receives air from the cold booster only in the event that a pumping point is imminent.

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) The invention will be described in further detail with reference to the figure, which shows a device for separating air by cryogenic distillation according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) The device comprises a system of columns comprising a column operating at a first pressure K1 and a column operating at a second pressure K2 below the second pressure. The columns are thermally connected through a bottom reboiler of the second column heated by nitrogen from the top of the first column. Nitrogen and oxygen enriched reflux flows, not shown, are sent from the column K1 to the column K2. Liquid oxygen 31 is extracted from the bottom of the second column K2 and gaseous nitrogen 33 is extracted from the top of the second column. Liquid nitrogen LIN is sent from the top of the second column in certain phases in order to help to keep the method cold. An oxygen rich fluid is sent to the exchanger E to be heated, for example, liquid oxygen 31 can vaporise in the heat exchanger E. A nitrogen rich fluid is sent to the exchanger E to be heated.

(4) The device comprises a first air expansion turbine T1, a second air expansion turbine T2, a first air booster C1 coupled to the first turbine and a second air booster C2 coupled to the second turbine.

(5) Compressed air 1 at a pressure P and originating from a main air compressor (MAC) is divided into two fractions, a first fraction 3 of which is sent to the heat exchanger E without having been compressed at a pressure above the pressure P. A second fraction 5 is sent to the first booster C1, where it is compressed at a pressure above the pressure (P) of the first fraction 3. The outlet of the first booster C1 is connected to the inlet of said booster by a duct 25 through a valve V8.

(6) According to a first variation, the first fraction 3 is cooled in the heat exchanger E to an intermediate temperature thereof and, having not been compressed in the first booster, is sent to the first and the second turbines through the open valve CL3 and the open valves V5, V13, V4, V19.

(7) The second fraction 5 cools in the heat exchanger E to an intermediate temperature thereof, after having been compressed in the first booster C1. It is subsequently sent to the second booster C2.

(8) During normal operation, expanded air originating from the first and second turbines is sent to the first column K1 in order to be separated through the valves V6, V15, V11 and the duct 13. The second fraction 5 is compressed in the second booster C2, passes through the open valve CL1 and is subsequently cooled in the heat exchanger before being sent in liquid form to the first column K1 through the valve V9. The valves V2 and V3 are closed.

(9) If the booster C1 approaches its pumping point, part of the boosted air is taken, after cooling in a cooler downstream of the booster, is allowed to expand by the valve V8 and is sent to the suction side of the booster C1.

(10) If the booster C2, supplied with air 19 originating from an intermediate point of the heat exchanger E, approaches its pumping point, none of the air boosted in the booster C2 is sent to the suction side of the booster C2. The booster C2 does not have any coolant downstream of the booster. If the flow boosted in C2 passes under a threshold indicating that the pumping point is imminent, part of the boosted air is sent via the duct 23, is allowed to expand in the valve V3 and reaches the suction side of the turbine T2 in order to be allowed to expand therein and to be sent to distillation.

(11) The detection threshold of the imminence of the pumping point is defined by defining a pressure drop threshold between two points of the booster, which threshold must not be exceeded. As long as the pressure drop remains below the threshold, all the boosted air is sent to the heat exchanger in order to be liquefied therein.

(12) Once the pressure drop has reached the threshold, the valve is opened that allows the air to pass to the turbine.

(13) The remainder of the boosted air is returned to the heat exchanger E through the valve CL1 and is at least partly liquefied in the exchanger, before being allowed to expand in the valve V9 and being sent to the column K1.

(14) Alternatively, the part of the air sent to the inlet of the turbine T2 can be sent to the outlet thereof arriving in the duct 17. In this case, the air expansion valve will allow this part of the air to expand to a pressure that is slightly above the pressure of the column K1.

(15) It is also possible for the part of the air to be sent to the inlet or the outlet of the turbine T1 instead of to the turbine T2. The air even can be sent to the two turbines T1, T2, to the inlets of the two turbines, to the outlets of the two turbines or to the inlet of one turbine and to the outlet of the other turbine.

(16) According to a second variation, the first fraction 3 is discharged from a heat exchanger at an intermediate temperature thereof and, having not been compressed in the first booster, is sent to the second booster C2.

(17) The second fraction 5 cools in the heat exchanger to an intermediate temperature thereof, after having been compressed in the first booster C1. It is subsequently sent to the first and the second turbines.

(18) Again, in this case, if the booster C2, supplied with air 19 originating from an intermediate point of the heat exchanger E, approaches its pumping point, none of the air boosted in the booster C2 is sent to the suction side of the booster C2. The booster C2 does not have any coolant downstream of the booster.

(19) If the flow boosted in C2 passes under a threshold indicating that the pumping point is imminent, part of the boosted air is sent via the duct 23, is allowed to expand in the valve V3 and reaches the suction side of the turbine T2, without passing through the exchanger E, in order to be allowed to expand in the turbine T2 and to be sent to distillation.

(20) The detection threshold of the imminence of the pumping point is defined by defining a pressure drop threshold between two points of the booster, which threshold must not be exceeded. This pressure difference is equivalent to the minimum flow of air in the booster, which minimum flow must not be passed under. As long as the pressure drop remains above the threshold, all the boosted air is sent to the heat exchanger in order to be liquefied therein.

(21) Once the pressure drop passes under the threshold, the valve is opened that allows the air to pass towards the turbine.

(22) It is also possible to trigger opening of the valve if the air flow in the booster passes under a threshold.

(23) The remainder of the boosted air is returned to the heat exchanger E through the valve CL1 and at least partly liquefies in the exchanger, before being allowed to expand in the valve V9 and being sent to the column K1.

(24) Alternatively, the part of the air sent to the inlet of the turbine T2 can be sent to the outlet thereof arriving in the duct 17. In this case, the air expansion valve will allow this part of the air to expand to a pressure slightly above the pressure of the column K1.

(25) It is also possible for the part of the air to be sent to the inlet or the outlet of the turbine T1 instead of to the turbine T2. The air even can be sent to the two turbines T1, T2, to the inlets of the two turbines, to the outlets of the two turbines or to the inlet of one turbine and to the outlet of the other turbine.

(26) An oxygen rich fluid is sent to the exchanger E to be heated, for example, liquid oxygen 31 can vaporise in the heat exchanger E. A nitrogen rich fluid is sent to the exchanger E to be heated.

(27) The invention is also applicable to the case where the device only comprises a single air turbine coupled to a cold booster.

(28) In this case, in normal operation the air is sent from the cold booster to the heat exchanger. The air then can directly enter the system of columns after being allowed to expand or otherwise can be at least partly sent to the single turbine.

(29) In the event that part of the boosted air liquefies in the heat exchanger and is allowed to expand in a valve V9 upstream of the system of columns, when the air flow boosted in the booster C1 passes under a threshold indicating that pumping is imminent, the flow of liquid passing through the valve V9 can be increased. This valve will then be designed with respect to this operating case.

(30) It is understood that the device can comprise a single cold booster and a single turbine, which may or may not receive air from the cold booster outside a pumping risk period.

(31) This invention is applicable to any method using a cold air booster in a device for separating air by cryogenic distillation. For example, it is applicable to the following methods: FR2943408, WO05064252, EP2831525, JP2015114083, JP54162678, EP1055894, EP2600090, JP2005221199, EP2963370, EP2963369, FR2913670, FR3033397, EP2458311, EP1782011, EP1711765, FR2895068, EP2489968, DE102011121314, EP1014020, FR2985305, DE102006027650, FR2861841, FR3010778, EP644388 and FR2721383.

(32) The inlet temperature of the air booster preferably is between 0 C. and 180 C., even between 60 C. and 180 C.

(33) 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.

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

(35) 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.

(36) 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.

(37) 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.

(38) 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.

(39) 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.