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1. A method of producing compressed nitrogen by low-temperature fractionation of air in a distillation column system having a high-pressure column and a low-pressure column and a high-pressure column top condenser and a low-pressure column top condenser, both in the form of condenser-evaporators, wherein
the low-pressure column has the lowest operating pressure of all the separating columns in the distillation column system, feed air is compressed in a main air compressor to an overall air pressure, forming a high-pressure overall air stream, the main air compressor is the only external energy-driven gas compressor which is used in the process, a first substream of the high-pressure overall air stream is decompressed to perform work and introduced into the distillation column system, a second substream of the high-pressure overall air stream is cooled down in a main heat exchanger and introduced at least partly in liquid form into the distillation column system, gaseous top nitrogen from the high-pressure column is at least partly liquefied in the high-pressure column top condenser, forming a first liquid nitrogen stream, a first substream of the first liquid nitrogen stream is applied as reflux liquid to the high-pressure column, gaseous top nitrogen from the low-pressure column is at least partly liquefied in the low-pressure column top condenser, forming a second liquid nitrogen stream, a first substream of the second liquid nitrogen stream is applied as reflux liquid to the low-pressure column, an internal compression nitrogen stream which is formed by a second substream of the first liquid nitrogen stream and/or a second substream of the second liquid nitrogen stream, in the liquid state, is brought to a product pressure higher than the operating pressure of the high-pressure column, the internal compression nitrogen stream which has been brought to the product pressure is warmed in the main heat exchanger and then drawn off as gaseous compressed nitrogen product stream under the product pressure, at least one oxygen-enriched product gas stream is drawn off from the distillation column system and warmed in the main heat exchanger and all oxygen-enriched product gas streams are drawn off from the distillation column system in the gaseous state,characterized in that
the exit pressure of the first substream of the high-pressure overall air stream in the work-performing decompression is equal to or higher than the operating pressure of the high-pressure column, the feed air is compressed in the main air compressor to an overall air pressure at least 5 bar above the operating pressure of the high-pressure column, and the internal compression nitrogen stream in the liquid state is brought to a product pressure between 15 and 100 bar. |
