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The main users of gaseous nitrogen are the chemical, the oil and gas processing branches of the industry where the technological processes entail stringent requirements regarding the parameters of the nitrogen (oxygen impurity 1-5 ppm; pressure 0.5-0.8 MPa) and the power required for obtaining it.Industrial production of nitrogen is effected in cryogenic air-fractionating apparatus (AFA). In Russia and other countries, nitrogen AFA are standardized; they operate with low air pressure (0.6-0.9 MPa) and single fractionating column and expansion engine driven by waste gas. For a long time, nitrogen AFA types AAzh-6 and A-8 (nitrogen output 5100 and 8000 m3/h, respectively) have been produced and operated, AFA with an output of 1200 to 15,000 m3/h of nitrogen are planned.An advantage of the standard nitrogen AFA is their simple operating principle and design, which make their production less laborious and simplifies their operation. However, their efficiency is poor because of the limited possibilities of the rectification process in one single-section column. When the pressure in the column is 0.85 MPa and the number of plates in it tends toward infinity, the maximal degree of nitrogen extraction (the limit fraction of production nitrogen out of the processed air fed to the fractionation plant as dry saturated vapor) does not exceed 0.44.The degree of nitrogen extraction can be increased by additional fractionation of the still liquid which in single-section fractionation contains more than 60% nitrogen and becomes waste gas after evaporation in the condenser-evaporator and expansion in the turbo-expansion engine. There exist two approaches to the solution of this problem. The first approach involves recompression of part of the waste gas at the hot or cold temperature level in an additional compressor, and reeirculation of this stream through the fractionating column which in this case becomes a double-section column [1, 2]. The proportion of the recirculation stream determining the degree of nitrogen extraction depends on the excess of refrigerating power produced by the AFA over the required power (in traditional nitrogen AFA, this excess is used for obtaining part of the product in the liquid state).The second approach envisages a more profound separation of the still liquid in the additional fractionation column, and it is also associated with recirculation and recompression. However, whereas in the former case only part of the vapors of the still liquid circulates (and the propelling agent is a compressor) and fractionation of these vapors occurs in the same column and at the same pressure, in the latter case liquid nitrogen circulates which is obtained from the still liquid by fractionation at reduced pressure in the additional column, and the propelling agent is a cryogenic pump [3][4][5]. Liquid nitrogen from the upper part of the additional column is extracted and used for reflux in the main fractionation column, thereby increasing the reflex ratio in it, and the oxygen-enriched liquid from the lower part ...

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