Cryogenic variable liquid production method

Abstract

A method of producing a liquid product stream, for example, a liquid nitrogen product stream, at a production rate that is selectively varied. This variation is produced in either a waste expansion or air expansion process by increasing the pressure and flow rate of the feed stream during periods in which a high rate of liquid production is desired without substantially increasing the pressure of the exhaust stream produced by a variable speed turboexpander. This increases the expansion ratio across the turboexpander and therefore the refrigeration supplied to increase liquid production. At the same time, the increase in flow rate prevents a decrease in the performance of the variable speed turboexpander.

We claim: 
     
         1 . A method of producing a liquid product stream at a production rate that is selectively varied, said method comprising:
 producing the liquid product stream as a result of a cryogenic rectification process which employs a distillation column for separating nitrogen from a feed stream comprising the nitrogen and oxygen after having been compressed, purified and cooled;   the cryogenic rectification process employing a refrigeration cycle that comprises partially cooling at least part of the feed stream within a main heat exchanger used within the cryogenic rectification process for cooling the feed stream, expanding the at least part of the feed stream in a variable speed turboexpander to generate an exhaust stream and introducing the exhaust stream into the distillation column;   selectively varying the production rate of the liquid product stream by varying feed stream pressure of the feed stream such that the expansion ratio across the variable speed turboexpander increases and consequently the refrigeration imparted to the cryogenic rectification process is also increased, thereby increasing the production rate of the liquid product stream and vice-versa; and   increasing flow rate of the feed stream during an increase of the feed stream pressure and vice-versa such that efficiency of the variable speed turboexpander remains substantially constant.   
     
     
         2 . The method of  claim 1 , wherein expansion work is dissipated by coupling the variable speed turboexpander to a variable speed generator. 
     
     
         3 . The method of  claim 1 , wherein during a low production rate of the liquid product stream, the feed stream is compressed with a base load compressor and is further compressed in a booster compressor, the booster compressor being a variable speed compressor, and during a high production rate of the liquid product stream, the flow rate of the feed stream through the base load compressor is increased so as to increase the speed of the variable speed turboexpander and therefore the speed of the booster compressor and the feed stream pressure. 
     
     
         4 . The method of  claim 1 , wherein the liquid product stream is part of a nitrogen-rich liquid stream produced in connection with the generation of reflux to the distillation column. 
     
     
         5 . The method of  claim 4 , wherein:
 the reflux for the distillation column is generated by condensing part of a nitrogen-rich vapor column overhead of the distillation column through indirect heat exchange with a stream of a liquid oxygen-rich column bottoms of the distillation column, thereby partially vaporizing the stream of the oxygen-rich liquid column bottoms to produce residual oxygen-rich liquid and the stream of the oxygen-rich vapor;   another part of the nitrogen-rich vapor column overhead is discharged as a nitrogen vapor product stream;   the stream of the oxygen-rich liquid is subcooled through indirect heat exchange with the oxygen-rich vapor stream and the nitrogen product stream and thereafter is expanded by an expansion valve prior to the indirect heat exchange with the part of the nitrogen-rich vapor column overhead; and   the oxygen-rich vapor stream and the nitrogen product stream after the indirect heat exchange involving the subcooling of the stream of the oxygen-rich liquid are fully warmed within the main heat exchanger.   
     
     
         6 . The method of  claim 5 , wherein:
 part of the feed stream is partially cooled and expanded within the turboexpander and another part of the feed stream is fully cooled and liquefied to produce a liquid feed stream; and
 the liquid feed stream is introduced into the distillation column.