Use of process analytics in cryogenic air separation plants – Case Study

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Courtesy of Siemens Industry, Inc. - Process Analytics

For further treatment of primary and intermediate materials, large amounts of oxygen and nitrogen are required by many industries, e. g. chemical and petrochemical, metal, semiconductor and food industry, in machine construction and many others. Oxygen and nitrogen are, together with Argon, the main constituents of ambient air. They are obtained from there by means of the air separation technology in highly automated air separation plants. Siemens is a leading supplier of Process Analytical Instrumentation and offers very capable gas analyzers which meet perfectly the demanding requirementsof air separation plants. Furthermore, Siemens offers complete analytical solutions from planning and engineering up to installation and maintenance. This Case Study explains the cryogenic air separation process and the related analysis tasks.

Air separation

The composition of dry air is approximately (by volume) 78% nitrogen, 21% oxygen, and 1% argon plus small amounts of noble gases, carbon dioxide, traces of hydrocarbons and other impurities. Oxygen, nitrogen and argon are required as industrial gases in large quantities and with a high degree of purity. Therefore, processes have been and are further developed to produce these gases through the separation of air. While there are variations in process details, all air separation plants employ one of two types of process technology:

  • Air separation at very low temperatures to liquefy the air and to produce the desired products by subsequent distillation (cryogenic process) based on differences in boiling points or
  • Air separation at higher pressure using adsorption effects based on differences in specific properties of the gases (pressure swing adsorption, PSA).

All cryogenic air separation processes from air as raw material to oxygen, nitrogen or noble gases as products consist of a similar series of steps independently whether large stand-alone plants or small and compact plants located directly at the consumers place are concerned. Variations reflect the desired product mix and individual priorities of the user. In all cases, process analyzers are required to control and optimize the process.

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