Process and Device for Obtaining Liquid Nitrogen by Low Temperature Air Fractionation

Abstract

A device for obtaining liquid nitrogen by low-temperature air fractionation in a distillation column system for nitrogen-oxygen separation includes a high-pressure column; a low-pressure column; a high-pressure column top condenser which is constructed as a condenser-evaporator and comprises a liquefaction compartment and an evaporation compartment; a low-pressure column top condenser which is constructed as a condenser-evaporator and comprises a liquefaction compartment and an evaporation compartment. A throttle stream is formed by one part of a purified feed air that is liquefied or pseudoliquefied in a main heat exchanger. The throttle stream is expanded, and at least some of the expanded throttle stream is introduced as refrigerant stream into the evaporation compartment of the high-pressure column top condenser.

Description

[0001]This U.S. patent application claims priority of European patent document 09012802.6 filed on Oct. 9, 2009, the entirety of which is incorporated herein by reference.FIELD OF INVENTION[0002]The present invention is directed to a process and device for obtaining liquid nitrogen by low-temperature air fractionation in a distillation column system.BACKGROUND OF INVENTION[0003]Liquid processes in which cold is transferred in a heat exchanger to a high-pressure air stream are known from EP 316768 A2 (FIG. 1); U.S. Pat. No. 5,660,059; or DE 102004046344. All of these processes comprise a conventional two-column system in which the high-pressure column top condenser (main condenser) is cooled by the bottoms liquid of the low-pressure column.[0004]A disadvantage of these known processes is the high preliminary liquefaction of the air introduced into the distillation column system. This leads to a decreased separation efficiency and thereby to a relatively high energy consumption of the...

AI Technical Summary

Benefits of technology

[0006]According to the present invention, a classical double column is replaced by two columns which both have a top condenser. An expanded throttle stream is introduced at least in part into the high-pressure column top condenser and there generates liquid nitrogen which can be applied as reflux to the high-pressure column and / or the low-pressure column and / or can be obtained directly as pressurized liquid product. In this manner, the cold contained in the throttle stream can be used particularly efficiently and a particularly low energy consumption results.

[0013]In column systems such as in U.S. Pat. No. 6,499,312, the high-pressure column top condenser is not cooled with a throttled airstream, but with bottoms liquid from the high-pressure column. In contrast, the present invention has the advantage that a fraction of constant composition (and therefore constant boiling temperature) is used on the evaporation side of the high-pressure column top condenser. In particular, under changing load (underload / overload) this gives particularly stable operation of the columns. Even if, under a change in load, the composition of the fractions in the columns changes, the top temperature of the high-pressure column remains constant and the operating pressures of the columns need not be adjusted. In addition, the liquid air from the throttle stream (approximately 21 mol % oxygen content) boils at a lower temperature than the bottoms liquid of the high-pressure column (minimum 32 mol %, generally 36 to 40 mol % oxygen content). Therefore, the operating pressure of the high-pressure column can be kept relatively low in the present invention and the process operates particularly favourably energetically.

[0018]The mechanical energy generated in the expansion machines may be transferred by mechanical coupling to two series-connected recompressors in which some of the air is further compressed from the intermediate pressure to the high pressure. The high-pressure stream can then be utilized as throttle stream; alternatively or in addition, the two turbine streams are formed by the high-pressure stream. In this case, the generation of cold and thereby the liquid production can be further increased, without energy needing to be supplied from the outside.

Problems solved by technology

A disadvantage of these known processes is the high preliminary liquefaction of the air introduced into the distillation column system.

This leads to a decreased separation efficiency and thereby to a relatively high energy consumption of the system.

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