Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-02-05
2002-07-16
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S646000, C062S925000
Reexamination Certificate
active
06418753
ABSTRACT:
The invention relates to a process for the low-temperature fractionation of air in which compressed and prepurified feed air is introduced into: a rectification system for nitrogen-oxygen separation, which rectification system has a pressure column, where at least a part of the compressed and prepurified feed air is fed to the pressure column and where an oxygen-enriched fraction is taken off from the pressure column and passed to a further working step within the rectification system.
Processes of this type are disclosed, for example, by Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering], 2nd edition 1985, Chapter 4 (pages 281 to 337). The rectification system for nitrogen-oxygen separation can be a one-column system having a single column, the pressure column in the context of the invention, a two-column system having a pressure column and a low-pressure column or a multi-column system. having further separation columns for nitrogen-oxygen separation. Hausen/Linde shows a plurality of examples of one-column systems (page 282, FIGS. 4.1. and 4.2.; page 287, FIG. 4.4; pages 329/330, FIGS. 4.30., 4.31. and 4.32.); the invention can be applied especially to a single column with top cooling via an oxygen-enriched liquid from the pressure column (Hausen/Linde, page 330, FIG. 4.31). Examples of two-column systems may also be found in Hausen/Linde (page 284, FIG. 4.3. and various examples in Sections 4.5.1 and 4.5.2). To generate ascending vapour for the low-pressure column, a part of the bottom liquid is evaporated in a condenser-evaporator (usually termed main condenser) which is operated, for example, with a gas fraction from the pressure column or with air as heating medium. The condenser-evaporator can be implemented by one or more heat-exchange blocks which can be operated, for example, as circulation evaporator and/or falling-film evaporator.
The rectification system for nitrogen-oxygen separation in the context of the invention in addition comprises heat exchangers such as, for instance, condenser-evaporators which are required to operate the separation column(s) for the nitrogen-oxygen separation (especially the main condenser of a double column or the top condenser of a single column). The process of the invention and the corresponding apparatus can, if required, have, outside the rectification system for nitrogen-oxygen separation, additional separation columns for producing further air constituents, for example noble gases such as argon, helium, neon, krypton or xenon (see Hausen/Linde, Chapter 4.5.4).
Customarily, the oxygen-enriched fraction is taken off from the bottom region of the pressure column before it is passed to a further working step within the rectification system. This further working step can be formed, for example, by the further separation in the low-pressure column of a double-column system or by an evaporation, for example in the top condenser of a one-column system. By this means, all of the low-volatility contaminants of the feed air which were not removed in the prepurification upstream of the introduction into the rectification system are transported on into the subsequent working step together with the oxygen-enriched fraction. (“Low-volatility contaminants” is here taken to mean feed-air components whose boiling point is higher than that of oxygen.)
Such low-volatility contaminants can accumulate further, especially in subsequent evaporation processes. Some of these low-volatility substances, in particular N
2
O, can precipitate as solids and must be removed from time to time to avoid a blockage of heat-exchange passages in the corresponding evaporators (for example in the main condenser of a double-column system). To eliminate the solids which have settled out, the entire plant must be shut down. In the case of a large air fractionation plant, this can mean an operational shutdown of, for example, two to five days. This problem is discussed in Wenning, Lachgas in Luftzerlegungsanlagen [laughing gas in air fractionation plants], Linde-Berichte aus Technik und Wissenschaft, 77/1998, 32-36. Here, and in U.S. Pat. No. 5,629,208 the solution proposed is to eject N
2
O using a relatively intense flushing of the liquid arising in the main condenser. However, it has proved to be the case that this measure is not sufficient in all cases to avoid the operationally highly undesirable shutdown of the air fractionation plant.
As a solution to this problem, various methods available to those skilled in the art seem suitable.
Firstly, a purification device could be used which removes the unwanted substances from the oxygen-enriched fraction. In the course of this, for example, all of the oxygen-enriched fraction (the bottom-phase liquid of the pressure column in the case of a double column) is conducted in liquid form via an adsorber to remove N
2
O. (Liquid adsorbers were previously used at the same point to remove acetylene.) This procedure solves the operational problems in the evaporator, but represents relatively high capital expenditure. In addition, the adsorber must be regenerated from time to time, which, even in the case of a switchable device, leads to further operational expenditure.
Secondly, U.S. Pat. No. 5,471,842 discloses ejecting low-volatility components as early as in the pressure column by withdrawing, at its bottom, a purge fraction in the liquid state and by taking off, above the air feed, the oxygen-enriched fraction to be processed further in the low-pressure column. The purge fraction here is brought in the liquid state to a very high pressure, vaporized in the main heat exchanger against highly compressed feed air, admixed to the feed air upstream of the air prepurification and passed with this feed air back into the pressure column. Although this method works, as specified in U.S. Pat. No. 5,471,842, for the ejection of CO
2
which is effectively retained in the prepurification molecular sieve, the problem of N
2
O is not mentioned in U.S. Pat. No. 5,471,842. The process there is unsuitable for reliable ejection of N
2
O (see article by Wenning, paragraph 6); under some circumstances blockage of the passages of the main heat exchanger can occur due to precipitated N
2
O, which necessitated heating of this apparatus.
In a modification of the process proposed in U.S. Pat. No. 5,471,842 for CO
2
ejection, it would be possible to remove from the process completely the purge fraction withdrawn in the bottom of the pressure column by discarding it, if appropriate after recovery of some of its cold. The purge fraction can, for example, be directly discarded in the liquid state by discharging it, after removing it from the pressure column, into the atmosphere, for example via an ejector. Alternatively, it can be vaporized and/or heated by indirect heat exchange with a heating medium and then discarded in the gaseous state. By this means, some of the energy which is present in the purge fraction in the form of cold is recovered. The vaporization should take place at a temperature high enough that precipitation of low-volatility contaminants is avoided, for example by introducing the liquid purge fraction into a residual gas fraction at medium temperature. Another possibility is the recovery of the cold in a heat exchanger having switchable passages (Revex). All of these methods can be expedient in certain plants, but have the disadvantage that the separation work performed on the purge fraction is lost and thus there is a high operating expenditure in the form of additional energy consumption.
The object therefore underlying the invention is to design a process of the type mentioned at the outset and a corresponding apparatus in such a manner that the operating expenditure in the overall process can be kept particularly low.
This object is achieved by the features of Patent claim
1
. In the process of the invention the purge fraction, which is formed by at least a part, preferably the whole, of the bottom liquid of the pressure column, is fed without prior vaporization to a device for removing N
2
O.
A
Pompl Gerhard
Voit Jürgen
Capossela Ronald
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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