US20080047298A1

US20080047298A1 - Process and apparatus for generating a pressurized product by low-temperature air fractionation

Info

Publication number: US20080047298A1
Application number: US11/735,171
Authority: US
Inventors: Horst Corduan; Ulrich Ewert; Gerhard Pompl
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2006-04-13
Filing date: 2007-04-13
Publication date: 2008-02-28
Also published as: EP1845324A1; TW200834025A; EP1845323A1; KR20070101794A; CN101063592A

Abstract

The process serves for generating a pressurized product by low-temperature air fractionation. Feed air (1) is compressed (2), purified (4), cooled (9) and fed to a distillation column system (12) for nitrogen-oxygen separation (11, 23). A liquid product stream (13) is taken off from the distillation column system (12) for nitrogen-oxygen separation, brought (14) in the liquid state to an elevated pressure (PIV) and at this elevated pressure (PIV) vaporized or pseudovaporized (9). The (pseudo)vaporized product stream (16) is fed (17) as pressurized product to a gas pressure reservoir (19) which has a variable pressure (PA). The elevated pressure (PIV) is varied. The elevated pressure (PIV) is varied as a function of the pressure (PA) of the gas pressure reservoir (19).

Description

The invention relates to a process for generating a pressurized product by low-temperature air fractionation by means of internal compression in which:

- feed air is compressed, purified, cooled and fed to a distillation column system for nitrogen-oxygen separation;
- a liquid product stream is taken off from the distillation column system for nitrogen-oxygen separation, brought in the liquid state to an elevated pressure and at this elevated pressure it is vaporized or pseudovaporized and
- the vaporized or pseudovaporized product stream is fed as pressurized product to a gas pressure reservoir which has a variable pressure.

Process and devices for low-temperature fractionation of air are disclosed, for example, by Hausen/Linde, Tieftemperaturtechnik [Low-temperature technology], 2nd edition 1985, chapter 4 (pages 281 to 337). A “distillation column system” comprises at least one separation column and also the condensers and evaporators assigned to the separation columns of the system. The distillation column system for nitrogen-oxygen separation of the invention can be constructed as a one-column system for nitrogen-oxygen separation, as a two-column system (for example as a classic Linde-twin-column system), or else as three-column or multiple column systems. In addition to the columns for nitrogen-oxygen separation, it can have other devices for producing other air components, in particular noble gases, for example argon production.
In an internal compression process, at least one of the products is taken off in the liquid state from one of the columns of the distillation column system or from a condenser connected to one of these columns, brought in the liquid state to an elevated pressure, vaporized or (at supercritical pressure) pseudovaporized in indirect heat exchange, for example with feed air or nitrogen, and finally obtained as gaseous pressurized product and fed to a take-off system which consists, for example, of a gas pressure reservoir. The pressure increase in the liquid can be carried out by any known measure. Generally pumps are used in this process. However, it is also possible to utilize a hydrostatic potential and/or the pressure build up vaporization in a tank.
Such internal compression processes are disclosed, for example, by DE 830805, DE 901542 (=U.S. Pat. No. 2,712,738/U.S. Pat. No. 2,784,572), DE 952908, DE 1103363 (=U.S. Pat. No. 3,083,544), DE 1112997 (=U.S. Pat. No. 3,214,925), DE 1124529, DE 1117616 (=U.S. Pat. No. 3,280,574), DE 1226616 (=U.S. Pat. No. 3,216,206), DE 1229561 (=U.S. Pat. No. 3,222,878), DE 1199293, DE 1187248 (=U.S. Pat. No. 3,371,496), DE 1235347, DE 1258882 (=U.S. Pat. No. 3,426,543), DE 1263037 (=U.S. Pat. No. 3,401,531), DE 1501722 (=U.S. Pat. No. 3,416,323), DE 1501723 (=U.S. Pat. No. 3,500,651), DE 2535132 (=U.S. Pat. No. 4,279,631), DE 2646690, EP 93448 B1 (=U.S. Pat. No. 4,555,256), EP 384483 B1 (=U.S. Pat. No. 5,036,672), EP 505812 B1 (=U.S. Pat. No. 5,263,328), EP 716280 B1 (=U.S. Pat. No. 5,644,934), EP 842385 B1 (=U.S. Pat. No. 5,953,937), EP 758733 B1 (=U.S. Pat. No. 5,845,517), EP 895045 B1 (=U.S. Pat. No. 6,038,885), DE 19803437 A1, EP 949471 B1 (=U.S. Pat. No. 6,185,960 B1), EP 955509 A1 (=U.S. Pat. No. 6,196,022 B1), EP 1031804 A1 (=U.S. Pat. No. 6,314,755), DE 19909744 A1, EP 1067345 A1 (=U.S. Pat. No. 6,336,345), EP 1074805 A1 (=U.S. Pat. No. 6,332,337), DE 19954593 A1, EP 1134525 A1 (=U.S. Pat. No. 6,477,860), DE 10013073 A1, EP 1139046 A1, EP 1146301 A1, EP 1150082 A1, EP 1213552 A1, DE 10115258 A1, EP 1284404 A1 (=US 2003051504 A1), EP 1308680 A1 (=U.S. Pat. No. 6,612,129 B2), DE 10213212 A1, DE 10213211 A1, EP 1357342 A1, DE 10238282 A1, DE 10302389 A1, DE 10334559 A1, DE 10334560 A1, DE 10332863 A1, EP 1544559 A1, EP 1585926 A1, or DE 102005029274 A1.
“Gas pressure reservoir” is here taken to mean any system which serves for buffering gaseous pressurized product and in particular has a buffering capacity which is sufficient to compensate for periodic take-off fluctuations or which is sufficient to compensate for temporary deficits or surpluses in production which occur during load changes. One example of periodic take-off fluctuations is the oxygen supply to a steelworks in which owing to the operation of the converters at regular intervals, high volumes of oxygen are required in the short-term. A further example is an air fractionation unit whose production is continuously adjusted to a current consumption, but the load (production rate) of the air fractionation unit cannot be changed at the same rate as the consumption and therefore temporary deficits or surpluses occur during the load adjustment. Generally, the buffering capacity of the gas storage reservoir should be sufficient to compensate for the deficits or surpluses in production occurring due to a typical change of the consumption (within minutes or seconds) in such a manner that the production of an air fractionation plant can follow the change in consumption, without the minimum or maximum permitted pressure limits of the product being infringed. The load adjustment time of a typical air fractionation unit for a change in load over the full load range of 70% to 100% is 30 minutes to 2 hours.
Since a “gas pressure reservoir” is associated with high capital costs, it will generally not be designed for all possible cases, but only for the take-off fluctuations typical during normal operation. Exceptional situations must be covered if appropriate by blowing off the product or by an additional supply (for example evaporator for cryogenic liquids).
“Gas pressure reservoir” is taken to mean in particular a system which has a buffering capacity which is at least equal to the amount of liquid product stream (pseudo)vaporizing to the pressurized product which the distillation column system generated in standard operation within a certain time period, for example at least equal to the amount generated within one minute, in particular at least equal to the amount generated within five minutes, or at least equal to the amount generated within 10 minutes. The buffering capacity of a gas pressure reservoir is determined by its volume and the possible width of variation of its pressure, that is to say the difference between the maximum and minimum operating pressure. The minimum operating pressure is established by the pressure requirements of the consumers, the maximum operating pressure by the construction of the gas pressure reservoir and safety regulations applicable thereto. A “gas pressure reservoir” can be formed, for example, by one or more dedicated gas pressure reservoir vessels or by a pipeline system having long piping lengths which serves, for example, for supplying a plurality of consumers with pressurized gas. Such a “gas pressure reservoir” is operated in a defined pressure range which is determined by a minimum permissible pressure and a maximum permissible pressure. Between these two values there is typically a difference of at least 2 bar, in particular at least 5 bar, preferably at least 10 bar. The larger the permissible width of variation of the pressure, the greater the available capacity in the pressure buffer of the gas pressure reservoir. The necessary capacity of the pressure buffer depends essentially on the course of the take-off fluctuations which are generally subject to a defined systematic change. In order to be able to flow into the gas pressure reservoir, the pressurized product obtained in the distillation column system must have a pressure which is higher than the pressure in the gas pressure reservoir. Hitherto this demand was met by the internal compression product being vaporized at a pressure which ensures introduction of the pressurized product into the gas pressure reservoir even at the maximum pressure of the gas pressure reservoir. The pressure during vaporization and also the operating pressures in the distillation column system are kept constant. In the case of a currently lower pressure in the gas pressure reservoir, the gaseous pressurized product is throttled, as a result of which energy is lost.

SUMMARY OF THE INVENTION

Thus, one aspect of the present invention is to provide a process of the type mentioned above which operates particularly expediently with respect to energy.
Upon further study of the specification and appended claims, further objects, aspects and advantages of this invention will become apparent to those skilled in the art.
In accordance with the invention, the elevated pressure (that is to say the pressure of the internal compression product) is varied and the elevated pressure (PIV) is varied as a function of the pressure (PA) of the gas pressure reservoir.
By adjusting the pressure of the internal compression product, the vaporization can take place at reduced pressure when the pressure in the gas pressure reservoir is below its maximum value. This means that less energy need be used for vaporizing the product stream.
In an internal compression process, generally a gaseous heat carrier stream is compressed to a high pressure (PW) and used at this high pressure for the (pseudo)-vaporization of the liquid product stream by indirect heat exchange. In the context of the invention it is expedient when in this case the high pressure (PW) and/or rate (MW) of the heat carrier stream is varied and the high pressure (PW) and/or rate (MW) is varied as a function of the pressure (PA) of the gas pressure reservoir. As a result, in the compression of the heat carrier stream, energy is saved when the pressure of the gas pressure reservoir is below its maximum value. In practice, the last-mentioned variation can be directed according to the pressure of the internal compression product (PIV); the said dependence on the pressure (PA) of the gas pressure reservoir is then an indirect one.
The heat carrier stream can be formed, for example, by a substream of the feed air or by a nitrogen stream from the distillation column system. Frequently, a substream of the feed air is recompressed, used as heat carrier stream, and subsequently introduced into the distillation column system for nitrogen-oxygen separation. “Rate” is taken to mean here the molar amount per unit time which is measured, for example, in Nm³/h.
In addition, or alternatively, in the context of the invention, energy can also be saved as a result of the fact that the cold generation at reduced pressure (PA) in the gas pressure reservoir is decreased by varying the amount of cold generated in the cold generation system of the process as a function of the pressure (PA) of the gas pressure reservoir.
The cold generation system can comprise one or more expansion machines for work-producing expansion of one or more process streams, one or more cold systems driven by external energy and or the supply of cold by one of more low-temperature liquid streams. Typically, in the invention, the rate of one or more process streams passed via an expansion turbine is controlled. At reduced pressure in the gas pressure reservoir, this is decreased. Correspondingly decreased demand for pressure energy leads to further energy savings.
In a further embodiment of the process according to the invention, one or more operating parameters of the distillation column system is varied as a function of the pressure (PA) of the gas pressure reservoir.
It is known to adjust the operating parameters of an air fractionation system to variable product rates via a load change system. Such a load change system can comprise feed-forward control, for example an ALC (automatic load change), or a multivariable control unit, for example an MPC (model predictive control). In the context of the invention it is advantageous to use such a system for improving the operating behaviour of the system when the internal compression pressure is varied and thereby to optimize the operating parameters of the distillation column system. The controlled adjustment of these operating parameters ensures consistency between the selected internal compression pressure and the operating point of the distillation and in addition avoids impermissible loading of the heat exchanger. An essential advantage of use of a load change system is the possibility of limiting the gradient of the internal compression pressure, that is to say the internal compression pressure does not follow the take-off pressure at any optional speed, but in a controlled manner. This can lead to an increased throttling or to blowing off of the product stream, in the event of rapid change of the take-off pressure in a transition phase, even in the process according to the invention. In contrast to conventional processes, such occurrences proceed only for a short time, however.
The load change system in this embodiment of the invention is constantly active and adjusts the preset value for the internal compression pressure to the current take-off pressure. The preset pressure value of the load change system forms the sum of the current take-off pressure and a preselected difference, in order to avoid unnecessary blowing off when the take-off pressure rises. Of course, this type of load control can be combined with a load change system for the product rates.
In addition, combination with a predictive pressure control of the gas pressure reservoir (for example a pipeline) is advantageous, as is described in EP 1542102 Al. In this case the pressure course in the gas pressure reservoir is determined on the basis of available information on the future need of the connected end consumers. This can be used in the context of the present invention for determining the preset pressure value for the load change system in order to avoid blowing off product as far as possible.
In a further embodiment of the invention, the elevated pressure (PIV) is only just above the instantaneous pressure (PA) of the gas pressure reservoir (19); in particular, the difference (PIV−PA) between these two pressures is constantly less than half, in particular less than one third, in particular less than one fifth, of the range of variation of the pressure of the gas pressure reservoir (19). The range of variation of the pressure of the gas pressure reservoir is taken to mean the difference between the maximum permissible pressure and the minimum permissible pressure of the gas pressure reservoir.
The invention moreover relates to an apparatus for generating a pressurized product by low-temperature air fractionation comprising:

- a distillation column system for nitrogen-oxygen separation,
- means for feeding compressed, purified and cooled feed air into the distillation column system for nitrogen-oxygen separation,
- means for taking off a liquid product stream from the distillation column system for nitrogen-oxygen separation,
- means for bringing the product stream in the liquid state to an elevated pressure (PIV),
- means for vaporizing or pseudovaporizing the product stream at the elevated pressure (PIV),
- means for feeding the (pseudo)vaporized product stream as pressurized product to a gas pressure reservoir (19),
- means for varying the elevated pressure (PIV), and
- a closed-loop or open-loop control unit which varies the elevated pressure (PIV) as a function of the pressure (PA) of the gas pressure reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:
FIG. 1 shows a roughly simplified plan of the process and the apparatus according to the example; and
FIG. 2 shows a diagram of the time course of the take-off pressure and the internal compression pressure.

DETAILED DESCRIPTION OF THE DRAWINGS

Air 1 is brought to a first pressure P1 in a main air compressor 2. The compressed air 3 is purified in a purification unit 4. The purified air 5 is branched into a first substream 6 and a second substream 7. The first air substream 6 is cooled in a main heat exchanger 9 to about dew point and flows via the lines 10 and 11 into the distillation column system 12 for nitrogen-oxygen separation which, in the example, has a high-pressure column, for example, at 5-15 bar, and a low-pressure column, for example, at 1.2-3 bar, which are in a heat-exchange relationship via a shared condenser-evaporator, called the main condenser. The air 11 is introduced into the high-pressure column in a virtually completely gaseous state.
In the distillation column system for nitrogen-oxygen separation 12, the air is fractionated into at least one oxygen-enriched product steam 13 and at least one nitrogen-enriched fraction (which is not shown). The product stream 13 has, for example, an oxygen content of 98 to 99.5 mol %. It is taken off in the liquid state, for example from the bottom of the low-pressure column or the evaporation space of the main condenser. In a pump 14 the liquid product stream 13 is brought to an elevated pressure PIV which is higher than the operating pressure of the distillation column from which it was taken off and is, for example 15 to 30 bar. The oxygen 15 is passed at the elevated pressure in liquid or supercritical state to the cold end of the main heat exchanger 9 and in the main heat exchanger is vaporized or pseudovaporized and warmed to about ambient temperature. Via an outlet valve 18, the product stream leaves the system as gaseous pressurized product 16, 18 and is introduced into a gas pressure reservoir 14 which, in the example, is constructed as a pipeline system. Via the pipeline system 19, the gaseous pressurized oxygen is finally delivered to a number n, which is in principle as many as is desired, of consumers V1 to Vn.
The pipeline system also serves as product buffer. Depending on the instantaneous take-off rate, the pressure of the gas pressure reservoir (at the point where line 17 joins) in the example can vary between a maximum permissible pressure of 30 bar and a minimum permissible pressure of 15 bar.
The heat required for the (pseudo)vaporization is supplied by a heat carrier stream 21 which is also termed internal compression air and is a part of the second air substream 7 which is recompressed in a secondary compressor 20 to a high pressure PW which is higher than the first pressure P1. The pressure P1 is, for example, 5-15 bar, and the pressure PW is, for example, 30 to 40 bar. This pressure in substream 21/22 is adjusted via the valve 8 and the guide vanes of the compressor 20. At this high pressure the internal compression air 22 flows through the main heat exchanger 9 to the cold end and in so doing is condensed or, at supercritical pressure, is pseudo-condensed, in indirect heat exchange with the (pseudo)-vaporizing oxygen 15. The internal compression air is expanded via a valve 30 and at 23 enters, in part liquefied state, the distillation column system 12 for nitrogen-oxygen separation.
Another part 25 of the second air substream 7/21 is passed out of the main heat exchanger at an intermediate temperature as a turbine stream. Its rate relative to the internal compression air is adjusted via the guide vanes of the turbine. The ratio of the rates of the first substream 6 and second substream 7/21 is set via an expansion valve 30 in substream 22.
The turbine air 25 is expanded to about the operating pressure of the high-pressure column in an expansion turbine 26. The expanded turbine air 27 is introduced together with the first substream 10 via line 11 into the high-pressure column of the distillation column system for nitrogen-oxygen separation 12. The turbine 26 in the example is a preferred element in a cold generation system of this unit, but other types of cold generation systems not requiring such a turbine, could be used.
In a conventional manner, the entire air fractionation unit would operate in a steady state and the pump 14 would continuously generate a pressure of somewhat more than the maximum take-off pressure of, for example, 30 bar. The adjustment to the current take-off pressure would be achieved solely by an appropriate throttling in valve 18. Even in the case of a varying product rate, in pump 14 only the rate of liquid product stream 13/15 would be set, but the pressure would remain constant.
With the invention, in contrast, the outlet pressure of pump 14 is adjusted to the instantaneous take-off pressure. The pump 14 is set to an outlet pressure, or elevated (PIV) pressure, which is about 0.5 to 2 bar above the instantaneous take-off pressure, or pressure (PA) of the gas pressure reservoir (14). A certain difference as a margin is logical in order that, even when the take-off pressure increases, the gaseous pressurized product 16 need not be blown off immediately via line 28 and valve 29. The corresponding fine adjustment is performed by valve 18 in which only a slight pressure decrease is performed, however.
Preferably, not only the stream rates but also the various pressures in the air fractionation unit, including the parameters of the separation process in the interior of the distillation column system 12 for nitrogen-oxygen separation are controlled by means of a central process control system (which is not shown) which is conducted by an automatic load change system. In this case, inter alia the valves 8 and 30 are activated which determine the rate and pressure of the internal compression air 22, the valve 24 for establishing the rate of the turbine air 25, the pump 14 for establishing the current rate of the oxygen product and valve 18 for fine adjustment of the product pressure to the take-off pressure. For the exceptional case that it is not possible to have the unit follow an increasing take-off pressure rapidly enough, the process control system can also intermittently close valve 18 and blow off the gaseous pressurized product into the atmosphere via the line 28 and the valve 29.
FIG. 2, in the upper part, shows an example of a time course of the take-off pressure PA and the internal compression pressure PIV qualitatively over a period of five hours plotted along the x axis.
The lower part of the diagram of FIG. 2 is the time course of the rate which is delivered by the gas pressure reservoir to the consumers (continuous line).
In the upper part of the diagram, a continuous line shows the course of the take-off pressure PA in the pressure reservoir or in the product pipeline of the gas pressure reservoir (the “pressure of the gas pressure reservoir”). The take-off pressure PA can range within the range of variation of the gas pressure reservoir pressure between a minimum operating pressure (min) and a maximum operating pressure (max). When the take-off rate increases (continuous line below) the take-off pressure PA falls (continuous line at top) and vice versa. The internal compression pressure PIV (the “elevated pressure”) shown as a dashed line at the top follows the course of the take-off pressure PA in principle at some distance and with delay. The difference PIV−PA is less than one third of the range of variation of the pressure of the gas pressure reservoir.
The internal compression pressure PIV cannot be changed as rapidly as desired, so that short-term blow off of product can also occur with the process according to the invention (see dashed line at the bottom in FIG. 2). The blow-off rate can be kept low, however, by the invention.
In a specific numerical example for delivery of pressurized oxygen to a steelworks, the minimum operating pressure (min) is 20 bar and the maximum operating pressure is 35 bar, the difference PIV−PA is below 2 bar, preferably in the range between 0.5 and 1 bar.
Of course, the invention may be applied to any other internal compression process, in particular to those having differing cold generation having one or more turbines, which blow air into the high-pressure column and/or the low-pressure column or expand a nitrogen-enriched fraction from one of the separation columns of the distillation column system 12.
The closed-loop control according to the invention can be further refined by evaluating information on the future consumption rates of the consumers V1 to Vn and obtaining therefrom a prediction of future values of the take-off pressure, for example according to the method described in EP 1542102 A1. The load change system can then move early the state of the air fractionation unit in a direction which corresponds to the internal compression pressure PIV required in the future. In this manner, a still better adjustment of the course of the internal compression pressure to the take-off pressure can be achieved, which contributes significantly to avoiding occasional blow-off of product.
The entire disclosures of all applications, patents and publications, cited herein and of corresponding European application No. EP 06007760.9, filed Apr. 13, 2006, are incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims

1. A process for generating a pressurized product by low-temperature air fractionation, said process comprising:

compressing (2), purifying (4), and cooling (9) feed air (1) and then feeding the feed air to a distillation column system (12) for nitrogen-oxygen separation (11, 23),

removing a liquid product stream (13) from said distillation column system (12) for nitrogen-oxygen separation, bringing (14) the liquid product stream (13), in the liquid state, to an elevated pressure (PIV), and at this elevated pressure (PIV) vaporizing or pseudovaporizing (9) the liquid product stream,

feeding (17) the vaporized or pseudovaporized product stream (16) as pressurized product to a gas pressure reservoir (19) which has a variable pressure (PA),

varying the elevated pressure (PIV), and

varying the elevated pressure (PIV) as a function of the pressure (PA) of the gas pressure reservoir (19).

2. A process according to claim 1, further comprising compressing (20) a gaseous heat carrier stream (7, 21, 22) to a high pressure (PW), and vaporization or pseudovaporization of said liquid product stream (13, 15) is performed by indirect heat exchange (9) with said heat carrier stream which is at the high pressure, wherein the high pressure (PW) and/or the rate (MW) of the heat carrier stream is varied and the high pressure (PW) and/or the rate (MW) of the heat carrier stream is varied as a function of the pressure (PA) of the gas pressure reservoir (19).

3. A process according to claim 2, wherein said gaseous heat carrier stream (7, 21, 22) is an air stream.

4. A process according to claim 1, further comprising compressing (20) a gaseous heat carrier stream (7, 21, 22) to a high pressure (PW), and vaporization or pseudovaporization of said liquid product stream (13, 15) is performed by indirect heat exchange (9) with said heat carrier stream which is at the high pressure, wherein the high pressure (PW) and/or the rate (MW) of the heat carrier stream is varied and the high pressure (PW) and/or the rate (MW) of the heat carrier stream is varied as a function of the elevated pressure (PIV) of the liquid product stream (13).

5. A process according to claim 4, wherein said gaseous heat carrier stream (7, 21, 22) is an air stream.

6. A process according to claim 3, wherein said gaseous heat carrier stream is a compressed substream of the feed air which after being subjected to heat exchange with said liquid product stream is introduced into the distillation column system for nitrogen-oxygen separation.

7. A process according to claim 1, wherein cold for the process is obtained in a cold generation system (26) and the amount of cold generated in the cold generation system (26) is varied as a function of the pressure (PA) of the gas pressure reservoir (19).

8. A process according to claim 7, wherein said cold generation system comprises a substream of the feed air which is subjected to heat exhange with said liquid product stream, expanded, and then introduced into the distillation column system for nitrogen-oxygen separation.

9. A process according to claim 1, wherein the operating parameters of the distillation column system are varied as a function of the pressure (PA) of the gas pressure reservoir (19).

10. A process according to claim 1, wherein the elevated pressure (PIV) is only just above the pressure (PA) of the gas pressure reservoir (19).

11. A process according to claim 10, wherein the difference (PIV−PA) between the elevated pressure (PIV) and the pressure (PA) of the gas pressure reservoir (19) is less than half the range of variation of the pressure of the gas pressure reservoir (19).

12. A process according to claim 10, wherein the difference (PIV−PA) between the elevated pressure (PIV) and the pressure (PA) of the gas pressure reservoir (19) is less than one third the range of variation of the pressure of the gas pressure reservoir (19).

13. A process according to claim 10, wherein the difference (PIV−PA) between the elevated pressure (PIV) and the pressure (PA) of the gas pressure reservoir (19) is less than one fifth the range of variation of the pressure of the gas pressure reservoir (19).

14. A process according to claim 1, wherein an air stream (7, 21, 22) is compressed (20) to a high pressure (PW) and the liquid product stream (13, 15) is vaporized or pseudovaporized by indirect heat exchange (9) with said air stream at high pressure.

15. A process according to claim 1, wherein said the distillation column system comprises a high-pressure column and a low-pressure column which are in a heat-exchange relationship via a shared condenser-evaporator.

16. A process according to claim 1, wherein the elevated pressure (PIV) is set to a pressure which is 0.5 to 2 bar above the current the pressure (PA) of the gas pressure reservoir (19).

17. An apparatus for generating a pressurized product by low-temperature air fractionation, said apparatus comprising:

a distillation column system for nitrogen-oxygen separation (12),

means (1, 3, 5, 6, 7, 10, 11, 21, 22, 23, 25, 27) for feeding compressed, purified and cooled feed air into said distillation column system (12) for nitrogen-oxygen separation,

means (13, 15) for taking off a liquid product stream from said distillation column system (12) for nitrogen-oxygen separation,

means (14) for bringing said liquid product stream in the liquid state to an elevated pressure (PIV),

means (9) for vaporizing or pseudovaporizing said liquid product stream at the elevated pressure (PIV),

means (16, 17) for feeding the vaporized or pseudovaporized product stream as pressurized product to a gas pressure reservoir (19),

means for varying said elevated pressure (PIV) and

a closed-loop or open-loop control unit which varies said elevated pressure (PIV) as a function of the pressure (PA) of the gas pressure reservoir (19).

18. An apparatus according to claim 17, wherein said the distillation column system comprises a high-pressure column and a low-pressure column which are in a heat-exchange relationship via a shared condenser-evaporator.