DE69004647T2 - Method and device for the low-temperature separation of air. - Google Patents

Description

The invention relates to the field of air separation processes and in particular to a process and apparatus for producing nitrogen, oxygen and/or argon from air, using liquid air as a heat exchange medium for the high pressure column cooler to provide a process with high energy efficiency.

Conventional cryogenic air separation processes involve filtering feed air to remove particulate matter before compressing the air to provide energy for separation. In most cases, the feed air stream is then cooled and passed through absorbents to remove impurities such as carbon dioxide and water vapor. The resulting stream is subjected to cryogenic distillation.

Cryogenic distillation or air separation involves feeding the high pressure air into one or more separation columns operating at cryogenic temperatures, where the air constituents, including oxygen, nitrogen, argon, and the rare gases, can be separated by distillation.

Cryogenic separation processes that use vapor and liquid contact depend on the differences in vapor pressure of the respective components. The component that has a higher vapor pressure, meaning it is more volatile or boils at a lower temperature, tends to concentrate in the vapor phase. The component that has a lower vapor pressure, meaning it is less volatile or boils at a higher temperature, tends to concentrate in the liquid phase.

The separation process in which heating of a liquid mixture takes place to enrich the volatile components in the vapor phase and the less volatile components in the liquid phase is called distillation. Partial condensation is a separation process in which a vapor mixture is cooled to enrich the volatile component or components in the vapor phase and simultaneously enrich the less volatile component or components in the liquid phase.

A process combining successive partial evaporation and condensation processes and involving countercurrent treatment of the vapor in liquid phases is called rectification or sometimes continuous distillation. The countercurrent contacting of the vapor and liquid phases is adiabatical and may involve integral or differential contact between the phases.

Devices used to carry out separation processes based on the principles of rectification to separate mixtures are often referred to as rectification, distillation or fractionation columns.

The term "column" used herein and in the claims refers to a distillation or fractionation column or zone. It may also be referred to as a contacting column or zone, wherein the liquid or vapor phases are contacted for the purpose of separating a fluid mixture in countercurrent. For example, this involves contacting the vapor and liquid phases on a series of vertically spaced column trays or plates, often perforated and grooved, extending crosswise to the column, perpendicular to the central axis. Instead of the column trays or plates, packing elements may be used to fill the columns.

The term "double column" as used here refers to a higher pressure column whose upper end is in heat exchange with the lower end of a lower pressure column.

As used herein, the term "standard air separation process or apparatus" means the process and apparatus described above as well as other air separation processes known to those skilled in the art.

The term "indirect heat exchange" as used herein and in the appended claims means that two fluid streams exchange heat with each other without physical contact or mixing of the fluids.

Historically, nitrogen, oxygen and/or argon have been produced using one of two basic process principles, including the single column process and the double column process.

WO 86/02148 describes an air separation process for producing nitrogen using a double column system, using air as a heat exchange medium. The process described in this patent application uses however, air is used exclusively as a heat supply medium for the low pressure column and not simultaneously as a cooling medium for the high pressure column. Furthermore, the process requires that all the treated air is directed to the bottom reheater of the low pressure column, which means that the heat exchanger (the bottom reheater) is very large. Furthermore, the air is only partially condensed in the process according to the above-mentioned patent application, which makes the process difficult to control and/or difficult to adjust correctly.

The method according to the invention can be used for energy-efficient production of nitrogen, oxygen and argon.

The core of the invention lies in the use of evaporated and liquefied air as a heating and cooling medium between the high-pressure and low-pressure columns. Previously, nitrogen was used.

The invention will be explained in detail with reference to nitrogen. However, it is to be understood that the invention is also applicable to the production of oxygen or argon. It is obvious to those skilled in the art how to optimize the temperature, pressure and other operating conditions in order to optimize the production of oxygen and/or argon as the primary product.

The particular advantage of using air for the heating and cooling medium is that less energy is required to condense air than to condense a nitrogen-rich stream. Since the main energy cost is in the compression of the gases, the lower pressure required to condense air at a given temperature is less costly than condensing nitrogen.

For example, nitrogen condenses at a pressure of 7 bar at -180ºC. In contrast, only a pressure of 6 bar at -178ºC is required to condense air. The temperature difference of 2ºC and the pressure difference of 1 bar are therefore responsible for the reduced energy consumption in the process according to the invention.

In state-of-the-art processes in which nitrogen is used as a heating and cooling medium between the high-pressure and low-pressure columns, it is necessary to compress the supplied air to the higher pressure required by the nitrogen. The primary energy saving therefore comes from the reduced requirement for compression of the supplied air.

The process according to the invention enables the production of high-purity nitrogen in the amount of more than 90% of the nitrogen contained in the initially supplied air. It can be produced in a pressure range between about 3 bar and about 15 bar. Both high-pressure and low-pressure nitrogen can be produced. This can be done separately or together. In addition, the process is highly energetically efficient compared to processes according to the state of the art.

In accordance with the invention there is provided a cryogenic process for producing nitrogen from air comprising the following steps:

A) dividing cooled, compressed supply air, which is substantially free of moisture and contaminants, into a first and a second partial flow of supply air;

B) introducing the first partial stream of supplied air into a high-pressure column equipped with a top condenser ;

C) separating the first partial stream of supplied air in the high-pressure column by means of cryogenic distillation into a first nitrogen-rich partial stream and a first oxygen-rich partial stream;

D) removing at least part of the first oxygen-rich substream from the high-pressure column;

E) introducing at least a portion of the first oxygen-rich substream into a low pressure column equipped with a bottom condenser/reheater and an overhead cooler/condenser for cryogenic separation into a second nitrogen-rich substream and a second oxygen-rich substream;

F) introducing the second partial stream of supplied air into the condenser/reheater in the low pressure column;

G) condensing the second partial stream of supplied air by indirect heat exchange with the second oxygen-rich partial stream in the low-pressure column, wherein at least a portion of the second oxygen-rich partial stream is evaporated;

H) introducing at least a portion of the condensed second partial stream of supplied air into the top condenser of the high-pressure column;

I) evaporating at least a portion of the second condensed partial stream of supplied air in the top condenser of the high-pressure column by indirect heat exchange with at least a portion of the first nitrogen-rich partial stream in the high-pressure column, to condense at least a portion of the first nitrogen-rich substream;

J) introducing at least a portion of the second partial stream of supplied air, which was evaporated by indirect heat exchange with the first oxygen-rich partial stream in the top condenser of the high-pressure column, into the low-pressure column for cryogenic separation, together with at least a portion of the first oxygen-rich partial stream, into a second nitrogen-rich partial stream and a second oxygen-rich partial stream;

K) discharging at least part of the second nitrogen-rich partial stream from the low-pressure column as useful stream;

L) removing at least part of the condensed second oxygen-rich partial stream from the low-pressure column;

M) introducing at least part of the withdrawn second oxygen-rich partial stream into the overhead cooler of the low-pressure column;

N) evaporating at least a portion of the second oxygen-rich partial stream in the overhead cooler by indirect heat exchange with at least a portion of the ascending second nitrogen-rich partial stream in the low-pressure column, wherein the second nitrogen-rich partial stream is condensed and forms a return stream for the low-pressure column; and

O) Discharging at least a portion of the evaporated second oxygen-rich partial stream from the overhead cooler as waste.

In one embodiment, the feed air is divided into two partial streams, one partial stream being directed to the bottom of a high pressure column and another partial stream being directed to a condenser/reheater located in the base of a low pressure column Good results have been obtained based on equal partial flows of supplied air, although other ratios can also be used.

In another embodiment, the feed air is divided into three partial streams. Two of the partial streams of feed air are directed to the high pressure column and the condenser/reheater at the base of the low pressure column, as described above. The third partial stream is expanded to cool the plant and introduced into the low pressure column for cryogenic separation.

In one embodiment, a portion of the condensed nitrogen-rich substream is separated in the high-pressure column and introduced into the low-pressure column to obtain an additional return stream. At the same time, the second substream of supplied air, which has been evaporated by an indirect heat exchange contact with nitrogen in the top condenser of the high-pressure column, is then introduced into the low-pressure column for cryogenic separation.

A portion of the second nitrogen-rich stream can be discharged as high pressure nitrogen feed stream, while the remaining portion is used to provide reflux for the low pressure column.

A portion of the high-pressure nitrogen useful stream can be expanded to cool the system and mixed with the low-pressure nitrogen useful stream.

The second oxygen-rich stream falling to the bottom of the high pressure column is vaporized by indirect heat exchange contact with the incoming second partial stream of supplied air, which is thereby condensed. In another embodiment, the second oxygen-rich partial stream also comprise a third feed stream which has been expanded before being introduced into the low-pressure column.

If desired, the waste oxygen can be expanded for equipment cooling. Alternatively, the waste oxygen, which has a purity of 70%, can be used as a product in applications where high purity oxygen is not required.

In accordance with another aspect of the invention, there is provided a system for producing nitrogen from cooled condensed air, comprising:

a high pressure distillation column equipped with a top condenser for cryogenic separation by fractionating cooled compressed feed air into a first nitrogen-rich sub-stream and a first oxygen-rich sub-stream;

a low pressure distillation column having an overhead condenser and a bottom reheater for fractionating cooled compressed feed air and at least a portion of the first oxygen-rich substream to obtain second nitrogen-rich and oxygen-rich substreams;

a line for supplying cooled compressed air to the reheater;

a line for transporting the part of the first oxygen-rich partial stream from the high-pressure column to the low-pressure column for the cryogenic separation taking place therein;

a line for transporting condensed supply air from the reheater to the overhead cooler;

a line for transporting vaporized air from the overhead cooler to the low pressure column;

a line for discharging the second oxygen-rich partial stream as waste after evaporation of the second oxygen-rich partial stream in the overhead cooler and a line for discharging the second nitrogen-rich partial stream from the low-pressure column as useful stream, and

a line for the direct introduction of cooled compressed air into the high pressure column.

Preferably, expansion devices are provided for expanding compressed air before introduction into the second distillation column, for expanding oxygen withdrawn from the overhead condenser of the second distillation column and/or for expanding a nitrogen product for the purpose of cooling.

Figure 1 shows a schematic flow diagram of the method and apparatus according to the invention for producing low pressure nitrogen;

Figure 2 shows a schematic flow diagram of a process and apparatus according to the invention similar to Figure 1, except that instead of waste expansion, air expansion is provided;

Figure 3 shows a schematic flow diagram of the method and the apparatus according to the invention for producing high-pressure and low-pressure nitrogen; and

Figure 4 shows a schematic flow diagram of the method and apparatus according to the invention similar to Figure 3, wherein a portion of the high pressure nitrogen is expanded to low pressure nitrogen.

Referring to the flow diagram of Figure 1, impurity-free compressed supply air is introduced into a heat exchanger 30 via a line 20. The air is introduced into the heat exchanger 30 preferably under a pressure in the range from about 5 bar to about 20 bar, the air temperature being cooled to cryogenic temperature by indirect heat exchange with discharged waste and useful streams.

Next, the feed air is divided into two substreams. Good results have been achieved with equal substreams or streams of feed air; however, other ratios may be used. The first substream of feed air is fed to the high pressure column 32 through lines 22 and 62, and the remaining second substream of feed air is fed to the reheater 58 of the low pressure column 34 through lines 22 and 60.

In the high pressure column 32, a pressure is preferably in the range of about 5 to 20 bar.

The first partial stream of supplied air is introduced into the lower part of the column 32 below the distillation tray as indicated by 36. Here, the first partial stream of supplied air is separated into a first nitrogen-rich vapor partial stream which rises to the top of the column 32 and a first oxygen-rich liquid partial stream which falls to the bottom of the column 32.

At least a portion of the first oxygen-rich liquid is discharged from the bottom of the high pressure column at 38. It consists of about 35% to about 40% oxygen, corresponding to about the same proportion as in prior art processes.

The first oxygen-rich liquid is discharged from the bottom of the high pressure column 32 via line 54, passed through the subcooler 46 where the temperature is further reduced by a useful nitrogen stream exiting from the top of the low pressure column 34 via line 48 and by a waste stream discharged through line 52 from the overhead condenser/evaporator 70 of the low pressure column 34.

The cooled first oxygen-rich liquid from the subcooler 46 is then introduced into the low pressure column 34 above the column bottom after being released through a valve 76.

The second partial stream of feed air entering the condenser/reheater 58 in the base of the low pressure column 34 is condensed by indirect heat exchange with an oxygen-rich liquid at the bottom of the low pressure column 34. This condenses the second partial stream of feed air and vaporizes the oxygen-rich liquid.

The condensed second partial flow of supplied air leaves the condenser/reheater 58 of the low pressure column 34 via a line 82, through which it enters the subcooler 46. The liquefied air exits the subcooler 46 via a line 84 and expands via a valve 44 into the condenser/reheater 40 of the high pressure column 32. If necessary, a portion of the condensed second partial flow of supplied air can be introduced into the low pressure column 34 via a line 90 after expansion via the valve 92 in order to control the air balance between the high pressure and low pressure columns.

The first nitrogen-rich vapor stream rises to the top of the high-pressure column 32, where it enters the condenser/reheater 40. There, the nitrogen stream is brought into indirect heat exchange contact with the condensed second partial stream of supplied air which enters the low pressure column 34 via the valve 44 from the condenser/reheater 58. This evaporates the liquefied air and condenses the nitrogen vapor. As shown in Figures 3 and 4, a portion of the condensed nitrogen stream is returned to the high pressure column 32 for the benefit of the required reflux.

Any nitrogen stream not condensed by indirect heat exchange with the condensed second substream of supplied air can be recovered as high pressure nitrogen by withdrawal from the upper part of the high pressure column 32, for example via a line 67 as shown in Figure 3.

A portion of the condensed nitrogen may be passed to the low pressure column 34 for additional reflux when the high pressure nitrogen flow is low or not required. This portion of the condensed nitrogen is discharged from the upper portion of the high pressure column 32 as shown in Figures 1 and 3 via line 68. The condensed nitrogen is then passed through the subcooler 66 where it is brought into indirect heat exchange contact with a discharged nitrogen useful and waste stream. From the subcooler 66, the condensed nitrogen is passed through an extension of line 68 and introduced into the low pressure column 34 after expansion via valve 78.

At the same time, the vaporized air discharged via line 56 from the condenser/reheater 40 at the top of the high pressure column 32 is separated by introduction into the low pressure column 34 via a line 64 at approximately the same level, as for the introduction of the first oxygen-rich liquid entering via line 54.

The first oxygen-rich liquid discharged from the base of column 32 and the vaporized air discharged from the condenser/reheater 40 at the top of high pressure column 32 via line 56 are further separated in column 34 into a second nitrogen-rich vapor substream and a second oxygen-rich substream.

The second nitrogen-rich vapor stream rises to the top of the low-pressure column 34, while the second oxygen-rich substream falls to the bottom of the low-pressure column 34.

A portion of the second oxygen-enriched liquid substream at the bottom of the low pressure column 34 is removed via a line 74 and passed through a first subcooler 46. There, the second oxygen-enriched liquid is further cooled by indirect heat exchange with a nitrogen gas removed via a line 48 from the upper part of the low pressure column 34 and with the waste stream exiting via a line 52 from the overhead condenser 70 of the low pressure column 34.

The second oxygen-enriched liquid is passed to a second subcooler 66 for further cooling by indirect heat exchange with a nitrogen gas discharged from the top of the high pressure column 32 via line 68 and with the waste oxygen stream exiting from the overhead condenser 70 through line 52.

The resulting cooled second oxygen-rich liquid is passed through an extension of line 74 where the liquid is fed into the overhead condenser 70 in the top of the low pressure column 34 after being released through a valve 72 is introduced to further cool the second oxygen-enriched stream.

A major portion of the second nitrogen-rich stream is recovered as a nitrogen feed stream from the upper portion of the low pressure column 34 via line 48. The gaseous nitrogen stream is passed through subcoolers 66 and 46 for warming before leaving the system.

The remaining portion of the second nitrogen-rich stream in the low pressure column 34 is condensed by heat exchange with the second oxygen-enriched liquid in the overhead evaporator/condenser 70 of the low pressure column 34, thereby vaporizing the second oxygen-enriched liquid. The condensation of the nitrogen provides reflux for the low pressure column 34. The vaporizing oxygen-enriched liquid exits the overhead evaporator/condenser 70 via line 52 and is subsequently heated as it passes through the subcoolers 66 and 46 and the heat exchanger 30.

After heating in the heat exchanger 30, the waste oxygen stream is passed through a turboexpander 78 where the stream can be expanded to cool the system.

The process described above clearly uses air as a heating and cooling medium between the high and low pressure columns. The nitrogen-rich stream has traditionally been used in prior art processes to transport heat to the bottom of the low pressure column. It should be noted that for a given nitrogen recovery, i.e. for the same composition of an oxygen-rich stream, more energy is required to condense the nitrogen-rich stream than to condense air. This means that for a given nitrogen recovery, the use of air as a heat transport medium the high pressure column can be operated at a lower pressure than is the case for the conventional prior art processes. For the same pressure in the high pressure column, the low pressure column can be operated at a higher pressure in accordance with the process according to the invention.

Table 1 shows the expected useful output of the process according to the invention shown in Figure 1 and described above for the production of nitrogen as useful electricity. Table 1 Total flow of supplied air Pressure of supplied air Useful nitrogen flow Nitrogen pressure Nitrogen purity (oxygen-rich) Waste stream Waste pressure Compressed air Column 32 Oxygen-rich liquid Condensed second partial flow of supplied air Line Head Bottom condensed second partial flow of supplied air evaporated second partial flow of air supplied from condenser/reheater 40 column 32 for nitrogen discharge subcooler 66 for condensed nitrogen discharge column 34 oxygen-rich liquid from column 34 oxygen-rich liquid discharged from cooler 66 oxygen-rich liquid after expansion useful nitrogen discharge column 34 oxygen waste stream from condenser 70 line valve

If one follows the embodiment shown in Figure 3 or 4, an air flow supplied at a pressure of 21 bar absolute generates a pressure of approximately 20 bar absolute within the high-pressure column 32 and a pressure of approximately 14 bar absolute within the low-pressure column 34.

Claims (17)

1. Cryogenic process for producing nitrogen from air, comprising the following steps: A) dividing cooled, compressed supplied air, which is substantially free of moisture and contaminants, into a first and a second partial stream of supplied air; B) introducing the first partial stream of supplied air into a high-pressure column (32) which is equipped with a top condenser (40); C) separating the first partial stream of supplied air in the high-pressure column (32) by means of low-temperature distillation into a first nitrogen-rich partial stream and a first oxygen-rich partial stream; D) removing at least part of the first oxygen-rich partial stream from the high-pressure column (32); E) introducing at least a portion of the first oxygen-rich substream into a low-pressure column (34) equipped with a bottom condenser/reheater (58) and an overhead cooler/condenser (70) for cryogenic separation into a second nitrogen-rich substream and a second oxygen-rich substream; F) introducing the second partial stream of supplied air into the condenser/reheater in the low pressure column (34); G) condensing the second partial flow of supplied air by indirect heat exchange with the second oxygen-rich partial flow in the low-pressure column (34), wherein at least a portion of the second oxygen-rich partial flow is evaporated; H) introducing at least a portion of the condensed second partial stream of supplied air into the top condenser (40) of the high pressure column (32); I) evaporating at least a portion of the second condensed partial stream of supplied air in the top condenser (40) of the high pressure column (32) by indirect heat exchange with at least a portion of the first nitrogen-rich partial stream in the high pressure column (32) in order to condense at least a portion of the first nitrogen-rich partial stream; J) introducing at least a portion of the second partial stream of supplied air, which was evaporated by indirect heat exchange with the first oxygen-rich partial stream in the top condenser (40) of the high-pressure column (32), into the low-pressure column (34) for cryogenic separation, together with at least a portion of the first oxygen-rich partial stream, into a second nitrogen-rich partial stream and a second oxygen-rich partial stream; K) discharging at least part of the second nitrogen-rich partial stream from the low-pressure column (34) as useful stream; L) discharging at least a portion of the condensed second oxygen-rich partial stream from the low-pressure column (34); M) introducing at least a portion of the withdrawn second oxygen-rich partial stream into the overhead cooler (70) of the low-pressure column (34); N) evaporating at least a portion of the second oxygen-rich partial stream in the overhead cooler (70) by indirect heat exchange with at least a portion of the ascending second nitrogen-rich partial stream in the low-pressure column (34), the second nitrogen-rich partial stream being condensed and forming a return stream for the low-pressure column (34); and O) discharging at least a portion of the evaporated second oxygen-rich partial stream from the overhead cooler (70) as waste. 2. The method according to claim 1, further comprising the following step: Discharging at least a portion of the condensed first nitrogen-rich partial stream from the high-pressure column (32) as high-pressure nitrogen useful stream. 3. The method of claim 1, further comprising the steps: Discharge of at least part of the condensed first nitrogen-rich partial stream from the high-pressure column (32) and Introducing at least a portion of the withdrawn first nitrogen-rich partial stream into the low-pressure column (34). 4. Method according to one of claims 1 to 3, further comprising the following steps: Further division of the compressed supplied air into a third partial flow of supplied air; expanding at least part of the third partial flow of supplied air for cooling purposes; and Introducing at least part of the expanded partial stream of supplied air into the low-pressure column (34). 5. The method of claim 2, further comprising the following step: Releasing at least a portion of the waste oxygen discharged from the overhead cooler (70) to cool the system. 6. The method of claim 2, further comprising the following step: Relaxing at least a portion of the high-pressure useful nitrogen stream before the outlet of the low-pressure useful nitrogen stream. 7. The method according to any one of claims 1 to 6, further comprising the following steps: Cooling of the supplied air through indirect heat exchange with waste and useful streams and Compressing the supplied air to generate a pressure between approx. 2 bar and approx. 20 bar in the high-pressure column (32). 8. Method according to one of claims 1 to 7, in which the first partial flow of supplied air in step B) is introduced into the lower half of the high-pressure column (32) and the first oxygen-rich partial stream in step D) is discharged from the base of the high-pressure column (32). 9. Method according to one of claims 1 to 8, in which the first oxygen-rich partial stream from step E) is introduced into the lower half of the low-pressure column (34) and the second oxygen-rich partial stream from step N) is discharged from the base of the low-pressure column (34). 10. The method according to any one of claims 1 to 9, further comprising the following steps: The waste oxygen obtained in step O) passes through a turbo expander (78) to be cooled, and the waste oxygen from the turbo expander (78) is heated by indirect heat exchange with the supplied air, which is thereby cooled. 11. Plant for generating nitrogen from cooled condensed air, with: - a high pressure distillation column (32) which is provided with a top condenser (40) for low temperature separation by fractionation of cooled compressed supplied air into a first nitrogen-rich partial flow and a first oxygen-rich partial flow; - a low pressure distillation column (34) with an overhead cooler (70) and a bottom reheater (58) for fractionating cooled compressed feed air and at least a portion of the first oxygen-rich substream to obtain second nitrogen-rich and oxygen-rich substreams; - a line for supplying cooled compressed air to the reheater (58); - a line (54) for transporting the part of the first oxygen-rich partial stream from the high-pressure column (32) to the low-pressure column (34) for the low-temperature separation taking place therein; - a line (82, 84) for transporting condensed supply air from the reheater (58) to the overhead cooler (40); - a line (64) for transporting evaporated air from the overhead cooler (40) to the low pressure column (34); - a line (52) for discharging the second oxygen-rich partial stream as waste after the evaporation of the second oxygen-rich partial stream in the overhead cooler (70) and a line (48) for discharging the second nitrogen-rich partial stream from the low-pressure column (34) as useful stream, and - a line (22, 62) for directly introducing cooled compressed air into the high-pressure column (32). 12. Plant according to claim 11, with a device (46) in the line (82, 84) which ensures complete condensation of the supplied air, and with a line (90) for introducing a portion of the condensed supplied air into the low-pressure column (34). 13. Plant according to claim 11 or 12, with a line (68) for removing the first nitrogen-rich useful stream from the high-pressure column (32) and for introducing it into the low-pressure column (34) in order to generate a return stream. 14. Plant according to one of claims 11 to 13, with: a compression device for compressing supplied air; a cleaning device for removing carbon dioxide, water vapor and other contaminants from the air compressed by the compression device; a heat exchanger (30) for cooling the compressed air from the cleaning device to a low temperature; a line connected to the overhead cooler (70) for introducing and discharging liquids and gases; a line (22, 60, 62) connected to the heat exchanger (30) and the high and low pressure column (32, 34) for introducing cooled compressed supplied air; and a valve in at least one of the lines to measure and release gases and liquids. 15. Plant according to one of claims 11 to 14, which further comprises an expander (78) and expands at least a portion of the useful nitrogen stream in order to cool it. 16. Plant according to one of claims 11 to 15, which further comprises an expander (78) for expanding waste oxygen. 17. Plant according to one of claims 11 to 16, further comprising an expander for expanding cooled compressed air before introduction into the low pressure column (34) in order to cool the air.