Process and device for low temperature air fractionation

This application is related to concurrently filed application “Low Temperature Air Fractionation With External Fluid” by Stefan Lochner, Attorney Docket No. LINDE-0678, claiming priority of DE 102007051183.5 filed Oct. 25, 2007, incorporated by reference herein.

The present invention relates to a process for low temperature air fractionation having a distillation column system for obtaining nitrogen.

Processes and devices for low temperature fractionation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik [Cryogenic engineering], 2nd edition 1985, chapter 4 (pages 281 to 337).

Examples of the type of air fractionation processes are described in EP 412793 B2, EP 773417 B1, EP 780648 B1, EP 807792 B1, EP 932004 A2 and US 2007204652 A1. It is known in such processes to withdraw a first oxygen-enriched residual fraction from the single column, vaporize it and expand at least a part of the vaporized residual fraction in an expansion machine in a work-producing manner in order to generate the refrigeration required to make up the exchange losses and, if appropriate, for product liquefaction. The first residual fraction is given off at the exit pressure of the expansion machine, that is customarily at approximately atmospheric pressure, and can then generally be used as only regeneration gas for an adsorption system for purifying the feed air. In SPECTRA processes, a second residual fraction can in this case be taken off from the single column together with the first residual fraction and at least in part vaporized; alternatively, the two residual fractions can be withdrawn at different points of the single column and vaporized separately from one another, for example in different passages of the top condenser of the single column.

Objects of this invention are to provide a process of the type mentioned above and a corresponding apparatus which may be operated with particularly favorable economics.

Upon further study of the specification to appended claims, other objects and advantages will become apparent. The figures in parentheses refer to the drawings to facilitate an understanding of the invention.

To achieve the process object there is provided a process for the low temperature air fractionation in a distillation column system for obtaining nitrogen which has a single column (12) in which

• • feed air (8) is cooled in a main heat exchanger (9) and introduced (11, 43) into the single column (12),
  • the single column (12) has a top condenser (13) in which vapour from the upper region of the single column is at least in part condensed,
  • a nitrogen product stream (15, 16, 17) is withdrawn from the upper region of the single column (12),
  • a first residual fraction (14, 19) is withdrawn in the liquid state from the single column (12), at least in part vaporized in the top condenser (13) and subsequently taken off from the top condenser as vaporized first residual fraction (19),
  • at least a first part (20) of the vaporized first residual fraction (19) is expanded in a work-producing manner in an expansion machine (21) and
  • a second residual fraction (18, 29) is withdrawn from the lower or intermediate region of the single column (12), recompressed (30) and subsequently passed (32) back at least to a first part of the single column (12), and
  • an improvement wherein
  • a second part of the vaporized first residual fraction (19) is not passed into the expansion machine (21) but is taken off as gaseous impure oxygen product (60) at about the inlet pressure of the expansion machine (21) and/or
  • a second part of the second residual fraction is taken off downstream of the recompression (30) as gaseous impure oxygen product (160).