Thermodynamic interlinkage of an air separation plant with a steam generator

In double fractionation of air at low temperatures in an air separation plant, comprising a fractionating column with a high pressure zone and an intermediate pressure zone and operating at elevated pressures to produce a relatively pure nitrogen product and a relatively pure oxygen products, wherein said high pressure zone operates at a pressure higher than 9 ata but lower than 25 ata, and said intermediate pressure zone operates at a pressure higher than 1.5 ata but lower than 9 ata, and wherein an incoming air stream is compressed to more than 9 ata as a main air feed before its separation, an improvement consisting essentially of thermodynamically interlinking said air separation plant with a steam generator and thereby carnotizing said steam generator, essentially by means of indirect heat exchanges between said nitrogen product and the heating fluids and the heated fluids of said steam generator, said indirect heat exchanges comprising:

• A. countercurrently and indirectly exchanging low-grade heat from said compressed main air feed to at least part of said nitrogen product;
• B. indirectly exchanging high-grade heat from said heating fluids of said steam generator to said at least part of said nitrogen product;
• C. work-expanding said at least part of said nitrogen product;
• and
• D. indirectly exchanging low-grade heat from said work-expanded said at least part of said nitrogen product to said heated fluids, thus recovering at least a substantial part of mechanical energy required for the compression of said incoming air stream without substantially impairing the efficiency of said steam generator.

In double fractionation of air at low temperatures and elevated pressures in a high-prssure zone and in an intermediate-pressure zone of an air separation plant to produce a relatively pure oxygen product and a relatively pure nitrogen product, after compression of said air, wherein said high pressure zone operates at a high pressure higher than 9 ata but lower than 25 ata and said intermediate-pressure zone operates at an intermediate pressure higher than 1.5 ata but lower than 9 ata, an improvement to recover most of the energy required for said compression by interlinking said air separation plant with a steam generator using flue gas for steam generation, consisting essentially of the following steps:

• A. forming a compressed main air feed by compressing said air to said high pressure, without intercooling thereof that would remove relatively low-temperature compression heat generated by said compressing, and dividing said compressed main air feed into a major part and a remaining part;
• B. transferring, by indirect heat exchange, said relatively low-temperature compression heat from said major part of said compressed main air feed to said nitrogen product so that the temperature of said nitrogen product is raised to form a hot nitrogen product;
• C. transferring, by indirect exchange therebetween, said relatively low-temperature compression heat from said remaining part of said compressed main air feed to at least one heated fluid feed for said steam generator;
• D. transferring, by indirect exchange therebetween, a quantity of high-temperature heat from said flue gas to said hot nitrogen product, so that the temperature of said hot nitorgen product is raised additionally and a very hot nitrogen product having a temperature well above 600.degree.C. is formed;
• E. work-expanding said very hot nitrogen product in a gas turbine to obtain mechanical energy therefrom and to cool said very hot nitrogen product so that a work-expanded nitrogen product having a lower temperature is formed, said mechanical energy being used for said compressing of said air in step A;
• and
• F. transferring, by indirect exchange therebetween, heat at said lower temperature from said work-expanded nitrogen product to at least one heated fluid feed to said steam generator to decrease said lower temperature to an exit temperature well below 150.degree.C. and to form a cooled nitrogen product having essentially no other contaminants than said relatively pure nitrogen product and without augmenting the volume of said nitrogen product by admixing it with said flue gas,
• whereby wastage of said relatively low-temperature compression heat generated by said compression is essentially avoided, a quantity of high-temperature heat is borrowed from said flue gas which is at a temperature substantially hotter than the critical temperature of steam, said high-temperature heat is used for production of mechanical energy, and essentially said quantity of heat is returned at said lower temperature to said at least one heated fluid, the efficiency of said steam generator not being substantially reduced.

In the simultaneous operation of a steam generator and an air separation plant as two separate and distinct closed systems, in which said steam generator has heated fluids as feeds therefor and a source of high-temperature heat generated by combusting an air-fuel mixture to form flue gases as combustion products and in which said air separation plant operates at elevated pressures on a compressed main air feed and produces a relatively pure nitrogen product, the improvement consisting essentially of the thermodynamic interlinking of said steam generator and said air separation plant through solely indirect heaat exchanges to produce mechanical energy by sequentially heating and work-expanding said nitrogen product without substantially impairing the efficiency of said steam generator, said indirect heat exchanges comprising:

• A. transferring, by indirect heat exchange therebetween, a quantity of relatively low-temperature compression heat from said compressed main air feed to said nitrogen product so that the temperature of said nitrogen product is raised to form a hot nitrogen product;
• B. transferring, by indirect heat exchange therebetween, a quantity of said high-temperature heat from said flue gases to said hot nitrogen product, so that the temperature of said hot nitrogen product is raised additionally and a very hot nitrogen product having a temperature well above 600.degree.C. is formed;
• C. after said work expanding said very hot nitrogen product in a gas turbine to obtain said mechanical energy therefrom so that a work-expanded nitrogen product having a lower temperature is formed, transferring, by indirect heat exchange therebetween, a quantity of heat at said lower temperature from said work-expanded nitrogen product to at least one of said heated fluids to decrease said lower temperature to an exit temperature below 150.degree.C. and to form a cooled nitrogen product having essentially no other contaminants than sad relatively pure nitrogen product and without augmenting the volume of said nitrogen product by admixing it with said flue gases.