Air separation method

Abstract

An air separation method in which a liquid air stream, produced by vaporizing a pumped liquid oxygen stream, is introduced into a lower pressure column and optionally, a higher pressure column of an air separation unit. The liquid air stream is subcooled by extracting a main air feed to the higher pressure column from a main heat exchanger at a temperature warmer than the liquid air stream to increase argon recovery in an argon column connected to the lower pressure column. This temperature is selected such that the liquid air stream approaches an average temperature of the return streams being fed into the main heat exchanger from the higher and lower pressure columns at a range between about 0.2K and about 3K.

1 . A method of separating air comprising:
 producing a first compressed and purified air stream and a second compressed and purified air stream having a higher pressure than the first compressed and purified air stream;   cooling the first compressed and purified air stream and the second compressed and purified air stream in a main heat exchanger, through indirect heat exchange with return streams produced in an air separation unit that include at least part of a pumped liquid oxygen stream, thereby to produce a main feed air stream and a liquid air stream;   introducing the main feed air stream into a higher pressure column of the air separation unit, expanding the liquid air stream and introducing at least part of the liquid air stream into a lower pressure column of the air separation unit;   introducing an argon-rich stream from the lower pressure column into an argon separation zone formed by-at least one column to produce an argon containing column overhead and an argon containing product stream composed of the argon containing column overhead;   subcooling a crude liquid oxygen stream composed of liquid column bottoms of the higher pressure column and a nitrogen-rich liquid stream composed of liquefied nitrogen column overhead of the higher pressure column and introducing at least part of the crude liquid oxygen stream and at least part of the nitrogen-rich liquid stream into the lower pressure column; and   the main feed air stream being extracted from the main heat exchanger at a temperature warmer than the liquid air stream and introduced into the higher pressure column at least at about said temperature, thereby subcooling the liquid air stream and increasing the liquid content thereof after having been expanded to improve the liquid to vapor ratio in the lower pressure column and thereby to increase the argon recovery, the temperature being selected such that the liquid air stream has an approach temperature approaching that of an average temperature of the return streams of no less than a range of between about 0.2K and about 3K, the average temperature being a calculated temperature at which a product of flow and enthalpy of the return streams at a cold end of the main heat exchanger is equal to the product of the flow and the enthalpy of the return streams at their actual temperatures.   
     
     
         2 . The method of  claim 1 , wherein the range is between about 0.4K and about 2K. 
     
     
         3 . The method of  claim 1 , wherein the temperature of the main feed air stream is in a range of between about 6K and about 25K warmer than the liquid air stream. 
     
     
         4 . The method of  claim 1 , wherein the temperature of the main feed air stream is in a range of between about 8K and about 15K warmer than the liquid air stream. 
     
     
         5 . The method of  claim 4 , wherein the range is between about 0.4K and about 2K. 
     
     
         6 . The method of  claim 1 , wherein:
 the liquid air stream is expanded to a pressure suitable for its introduction into an intermediate location of the higher pressure column;   the liquid air stream is divided into a first subsidiary liquid stream and a second subsidiary liquid stream;   the first subsidiary liquid stream is introduced into the higher pressure column; and   the second subsidiary liquid stream is expanded and introduced into the lower pressure column above a point of discharge of the argon-rich stream to the argon column.   
     
     
         7 . The method of  claim 1 , wherein:
 a third compressed and purified air stream is produced;   the third compressed and purified air stream is partially cooled within the main heat exchanger and introduced into a turboexpander to produce an exhaust stream for generation of refrigeration; and   the exhaust stream is introduced into the lower pressure column.   
     
     
         8 . The method of  claim 5 , wherein:
 a fourth compressed and purified air stream is produced by extracting the fourth compressed and purified air stream from an intermediate stage of a compressor used in forming the second compressed and purified stream; and   the fourth compressed and purified stream is expanded within another turboexpander and combined with the first compressed and purified air stream within the main heat exchanger to increase liquid production.   
     
     
         9 . The method of  claim 1 , wherein a nitrogen column overhead stream composed of the nitrogen column overhead is partially warmed within the main heat exchanger, expanded within a turboexpander to produce an exhaust stream for generation of refrigeration and the exhaust stream is introduced into the main heat exchanger and fully warmed therein. 
     
     
         10 . The method of  claim 1  or  claim 5  or  claim 6  or  claim 7  or  claim 8  or  claim 9 , wherein the liquid air stream is introduced into a liquid turbine to expand said liquid air stream to the pressure suitable for its introduction into an intermediate location of the higher pressure column. 
     
     
         11 . The method of  claim 1 , wherein the crude liquid oxygen stream and the nitrogen-rich liquid stream are subcooled through indirect heat exchange with the return streams that are formed from a nitrogen-rich vapor stream composed of column overhead of the lower pressure column and a waste vapor stream enriched in nitrogen to a lesser extent than the nitrogen-rich vapor stream, the nitrogen-rich vapor stream and the waste vapor stream being introduced into the main heat exchanger after having subcooled the crude liquid oxygen stream and the nitrogen-rich liquid stream. 
     
     
         12 . The method of  claim 1 , wherein:
 a first part of the crude liquid oxygen stream is expanded and introduced into the lower pressure column and a second part of the crude liquid oxygen stream indirectly exchanges heat with an argon column overhead stream composed of the argon column overhead, thereby condensing the argon column overhead stream and partially vaporizing the second part of the crude liquid oxygen stream;   liquid and vapor fraction streams resulting from partial vaporization of the crude liquid oxygen stream are introduced into the lower pressure column; and   part of the argon column overhead stream after having been condensed forms the argon product stream and a remaining part thereof after condensation is returned to the argon separation zone as reflux.