Distillation method and apparatus

Abstract

A distillation method and apparatus having application to the distillation of air in which liquid production make is varied by varying the pressure ratio across a turboexpander used in generating refrigeration. The pressure ratio is varied by varying the pressure of a compressed stream fed to the turboexpander. This is done by solely compressing such compressed stream by a first booster compressor during a low rate of production of liquid products. During a high rate of production of liquid products, the compressed stream is also compressed within a second booster compressor. The second booster compressor is driven by a variable speed drive to allow a variety of liquid production rates between the low level of liquid production and the high level of liquid production.

1 . A distillation method comprising:
 forming a first compressed stream and a second compressed stream containing oxygen and nitrogen, the first compressed stream and the second compressed stream containing oxygen and nitrogen;   fully cooling the first compressed stream in a main heat exchanger;   generating refrigeration by further compressing the second compressed stream, partially cooling the second compressed stream within the main heat exchanger and expanding the second compressed stream after having been partially cooled in a turboexpander to produce an exhaust stream;   introducing the first compressed stream and the exhaust stream into at least one distillation column configured to separate the nitrogen from the oxygen and to produce at least one liquid product stream enriched in one of the nitrogen and the oxygen;   during a low rate of production of the at least one liquid product stream, the second compressed stream being formed by compressing part of the compressed feed stream within a first booster compressor coupled to the turboexpander;   during a high rate of production of the at least one liquid product stream, the second compressed stream being formed by coupling a second booster compressor to the first booster compressor so that the part of the compressed feed stream is compressed to a pressure higher than that obtainable by the first booster compressor, thereby to increase the expansion ratio across the turboexpander and the refrigeration generated by the turboexpander;   driving the second booster compressor with a variable speed drive; and   varying the speed of the variable speed drive to vary the pressure of the part of the feed stream and therefore production of the at least one liquid product stream during the high rate of production.   
   
   
       2 . The method of  claim 1 , wherein:
 a feed stream composed of air is compressed in a base load compressor to produce a compressed feed stream;   the compressed feed stream is purified of higher boiling contaminants comprising carbon dioxide, water vapor and hydrogen carbons within a purification unit to form a purified compressed feed stream; and   the second compressed stream is formed from part of the purified compressed feed stream.   
   
   
       3 . The method of  claim 2 , wherein:
 the at least one distillation column comprises a double column unit having a higher pressure column in a heat transfer relationship with a lower pressure column such that a nitrogen-rich column overhead of the higher pressure column is condensed against boiling an oxygen-rich liquid of the lower pressure column;   the higher pressure column and the lower pressure column being connected such that a stream of a crude liquid oxygen column bottoms of the higher pressure column is expanded and introduced into the lower pressure column, streams of nitrogen-rich liquid produced from the condensation of the nitrogen-rich column overhead, at least in part, reflux both the higher pressure column and the lower pressure column;   the exhaust stream is introduced into the higher pressure column;   the first compressed stream is introduced into at least one of the higher pressure column and the lower pressure column; and   the at least one liquid product stream is at least one of a liquid oxygen product stream composed of the oxygen-rich liquid column bottoms and a liquid nitrogen product stream composed of part of one of the streams of the nitrogen-rich liquid used in refluxing the lower pressure column.   
   
   
       4 . The method of  claim 3 , wherein:
 a stream of the oxygen-rich liquid column bottoms is pumped to form a pumped liquid oxygen stream;   part of the pumped liquid oxygen stream forms the liquid oxygen product stream;   a remaining part of the pumped liquid oxygen stream is vaporized within the main heat exchanger;   the first compressed stream is formed by further compressing a remaining part of the purified compressed feed stream; and   the first compressed stream is introduced into the main heat exchanger to vaporize the remaining part of the pumped liquid oxygen stream.   
   
   
       5 . The method of  claim 3  or  claim 4 , wherein: the one of the streams of the nitrogen-rich liquid is subcooled;
 after having been subcooled, the part of the one of the streams of the nitrogen-rich liquid forms the liquid nitrogen product stream and a remaining part thereof is introduced into the lower pressure column as the reflux;   a waste nitrogen stream and a gaseous nitrogen product stream are withdrawn from the lower pressure column;   the waste nitrogen stream and the gaseous nitrogen product stream are passed in indirect heat exchange with the one of the streams of the nitrogen-rich liquid, thereby to subcool the one of the streams of the nitrogen-rich liquid;   the waste nitrogen stream and the gaseous nitrogen product stream are introduced into the main heat exchanger; and   the waste nitrogen stream and the gaseous nitrogen product stream fully warm within the main heat exchanger.   
   
   
       6 . A distillation apparatus comprising:
 at least one compressor compressing a feed stream containing oxygen and nitrogen and producing, at least in part, a first compressed stream and a second compressed stream;   a main heat exchanger in flow communication with the at least one compressor and configured such that the first compressed stream fully cools within the main heat exchanger;   a turbine booster compression system interposed between the main heat exchanger and the at least one compressor such that the second compressed stream is further compressed and introduced into the main heat exchanger, the main heat exchanger also configured such that the second compressed stream after having further been compressed partially cools within the main heat exchanger;   a turboexpander connected to the main heat exchanger such that the second compressed stream is expanded in the turboexpander to produce an exhaust stream, thereby to generate refrigeration;   at least one distillation column connected to the main heat exchanger to receive the first compressed stream and the exhaust stream in and configured to separate the nitrogen from the oxygen and to produce at least one liquid product stream enriched in one of the nitrogen and the oxygen;   the turbine booster compression system having a first booster compressor coupled to the turboexpander, a second booster compressor and a flow control network having valves operable to be set into positions such that at a low rate of production of the at least one liquid product stream, part of the compressed feed stream is compressed within the first booster compressor, thereby to further compress the second compressed stream and during a high rate of production of the at least one product stream, the second booster compressor is coupled to the first booster compressor so that the part of the compressed feed stream is compressed to a pressure higher than that obtainable by the first booster compressor, thereby to further compress the second compressed stream and to increase the expansion ratio across the turboexpander and the refrigeration generated by the turboexpander; and   a variable speed drive driving the second booster compressor such that varying the speed of the variable speed drive varies the pressure of the second compressed stream and therefore production of the at least one liquid product stream during the high rate of production.   
   
   
       7 . The distillation apparatus of  claim 6 , wherein:
 the at least one compressor comprises a base load compressor to compress a feed stream composed of air, thereby to produce a compressed feed stream; and   a purification unit is connected to the base load compressor, the purification unit configured to purify the compressed feed stream of higher boiling contaminants comprising carbon dioxide, water vapor and hydrogen carbons.   
   
   
       8 . The distillation apparatus of  claim 7 , wherein:
 the at least one distillation column comprises a double column unit having a higher pressure column in a heat transfer relationship with a lower pressure column such that a nitrogen-rich column overhead of the higher pressure column is condensed against boiling an oxygen-rich liquid of the lower pressure column;   the higher pressure column and the lower pressure column being are connected such that a stream of a crude liquid oxygen column bottoms of the higher pressure column is expanded and introduced into the lower pressure column, streams of nitrogen-rich liquid produced from the condensation of the nitrogen-rich column overhead, at least in part, reflux both the higher pressure column and the lower pressure column;   the double column unit is connected to the main heat exchanger so that the exhaust stream is introduced into the higher pressure column and the first compressed stream is introduced into at least one of the higher pressure column and the lower pressure column; and   the at least one liquid product stream is at least one of a liquid oxygen product stream composed of the oxygen-rich liquid column bottoms and a liquid nitrogen product stream composed of part of one of the streams of the nitrogen-rich liquid used in refluxing the lower pressure column.   
   
   
       9 . The distillation apparatus of method of  claim 8 , wherein:
 a pump connected to the lower pressure column to pump a stream of the oxygen-rich liquid column bottoms, thereby to form a pumped liquid oxygen stream;   the main heat exchanger is connected to the pump so that part of the pumped liquid oxygen stream forms the liquid oxygen product stream and a remaining part of the pumped liquid oxygen stream is vaporized within the main heat exchanger;   the at least one compressor also comprises a product boiler compressor interposed between the main heat exchanger and the base load compressor to further compress a remaining part of the compressed feed stream and thereby form the first compressed stream; and the first compressed stream is sufficiently compressed by the product boiler compressor to vaporize the remaining part of the pumped liquid oxygen stream within the main heat exchanger.   
   
   
       10 . The method of  claim 8  or  claim 9 , wherein:
 a subcooling unit is positioned to subcool the one of the streams of the nitrogen-rich liquid prior to being introduced into the lower pressure column as the reflux;   the subcooling unit is connected to the lower pressure column such that after having been subcooled, the part of the one of the streams of the nitrogen-rich liquid forms the liquid nitrogen product stream and a remaining part thereof is introduced into the lower pressure column as the reflux;   the subcooling unit is also connected to the lower pressure column such that a waste nitrogen stream and a gaseous nitrogen product stream pass in indirect heat exchange with the one of the streams of the nitrogen-rich liquid; and   the main heat exchanger connected to the subcooling unit and configured such that the waste nitrogen stream and the gaseous nitrogen product stream are introduced into the main heat exchanger.