Nitrogen liquefier retrofit for an air separation plant

Abstract

A method is disclosed for increasing liquid production involving retrofitting an existing air separation plant with a nitrogen liquefier. The nitrogen liquefier liquefies a nitrogen-rich vapor stream withdrawn from the higher pressure column to return a nitrogen-rich liquid stream to the higher pressure column. This increases liquid nitrogen reflux to the higher pressure column to in turn increase the production of liquid oxygen containing column bottoms of the higher pressure column and therefore, the production of oxygen-rich liquid in the lower pressure column. The increased production of the oxygen-rich liquid allows a liquid oxygen product to be taken at an increased rate or for the liquid oxygen product to be taken in the first instance, if the plant is not designed to produce such a product. Also liquid nitrogen and argon products can be produced at an increased rate as a result of the retrofit.

1 . A method of retrofitting an existing air separation plant to produce or to increase production of at least one liquid product:
 separating air within existing air separation plant having at least higher and lower pressure columns operatively associated with one another in a heat transfer relationship;   retrofitting the existing air separation plant by connecting a nitrogen liquefier to the higher pressure column, the nitrogen liquefier having no components in common with existing components of said existing air separation plant;   the nitrogen liquefier being connected to the higher pressure column such that the nitrogen liquefier only receives a nitrogen-rich vapor stream from a top portion of the higher pressure column, the nitrogen-rich vapor stream is liquefied in the nitrogen liquefier to produce a nitrogen-rich liquid stream and at least a portion of the nitrogen-rich liquid stream is introduced into the higher pressure column, thereby to increase liquid nitrogen reflux to the higher pressure column, production of a crude liquid oxygen column bottoms formed in the higher pressure column and therefore, an oxygen-rich liquid formed in a bottom region of the lower pressure column; and   withdrawing the at least one liquid product from the air separation unit, the at least one liquid product comprising an oxygen-rich liquid stream composed of the oxygen-rich liquid.   
   
   
       2 . The method of  claim 1 , wherein, within the nitrogen liquefier:
 a nitrogen vapor stream comprising the nitrogen-rich vapor stream is warmed within a heat exchanger, expanded to exhaust stream pressure of a turbine exhaust stream and combined with the turbine exhaust stream to produce a combined stream;   the combined stream is compressed in a recycle compressor and after removal of the heat of compression is divided into a refrigerant fluid stream and a remaining part of the combined stream;   the refrigerant fluid stream is compressed in the booster compressor, partially cooled in the heat exchanger and then introduced into the turboexpander to generate the turbine exhaust stream;   the turbine exhaust stream is warmed within the heat exchanger and combined with the nitrogen-rich vapor stream;   the remaining part of the combined stream is cooled within the heat exchanger and expanded to higher pressure column pressure; and   the nitrogen-rich liquid stream is formed at least in part from the combined stream.   
   
   
       3 . The method of  claim 2 , wherein:
 the work of expansion of the turboexpander powers the booster compressor;   the expansion of the remaining part of the combined stream produces a two-phase stream;   liquid and vapor phases of the two-phase stream are disengaged to form a vapor phase stream and a liquid phase stream;   the vapor phase stream is combined with the nitrogen-rich vapor stream to form the nitrogen vapor stream prior to its introduction into the heat exchanger; and   the liquid nitrogen stream is composed of the liquid phase stream.   
   
   
       4 . The method of  claim 1 , wherein a liquid nitrogen product stream is withdrawn that is made up of a further part of the nitrogen-rich liquid stream. 
   
   
       5 . The method of  claim 4 , wherein:
 the air separation unit also has an argon column connected to the lower pressure column to purify an argon-rich stream and thereby to produce an argon product stream; and   the further part of the nitrogen-rich stream is withdrawn at a rate that does not increase oxygen concentration within the argon-rich stream.   
   
   
       6 . The method of  claim 1 , wherein:
 the air separation unit also has an argon column connected to the lower pressure column to purify an argon-rich stream and thereby to produce an argon product stream;   the argon recovery is increased by increased production of the oxygen-rich liquid and removal of the oxygen-rich liquid stream.   
   
   
       7 . The method of  claim 1 , wherein the liquefier is operated intermittently so that the at least one liquid product stream is able to be stored for future utilization. 
   
   
       8 . The method of  claim 1 , wherein the existing air separation plant is configured such that attachment points exist within the higher pressure column of the existing air separation plant for connection of the nitrogen liquefier.