Method and system for providing auxiliary refrigeration to an air separation plant

Abstract

A method and system for cryogenic air separation that employs both a primary refrigeration circuit and an auxiliary refrigeration circuit is provided. The auxiliary refrigeration circuit is configured in a manner that it can be easily tied-in or modified to an existing air separation plant.

What is claimed is: 
     
         1 . A method of separating air in an air separation unit comprising a main heat exchanger configured to cool a compressed and purified feed air stream to a temperature suitable for the rectification and a distillation column system configured to rectify the compressed, purified and cooled air stream to produce at least one liquid product stream, the method comprising the steps of:
 compressing and purifying a feed air stream to produce the compressed and purified feed air stream;   diverting a first portion of the compressed and purified feed air stream to a first refrigeration circuit configured to produce a first cooled refrigeration stream;   diverting a second portion of the compressed and purified feed air stream to the main heat exchanger to cool the second portion of the compressed and purified feed air stream and wherein the cooled second portion of the compressed and purified feed air stream is subsequently directed to the higher pressure column of the distillation column system;   diverting a third portion of the compressed and purified feed air stream to a booster air compression circuit configured to produce a further compressed feed air stream and wherein part of the further compressed feed air stream is directed to the main heat exchanger where the further compressed feed air stream is cooled to produce a liquid air stream that is directed to the distillation column system;   diverting a fraction of the further compressed feed air stream from the booster air compression circuit to an auxiliary refrigeration circuit configured to produce a second refrigeration stream, the auxiliary refrigeration circuit comprising a second turbo-expander and an auxiliary heat exchanger;   diverting a fourth portion of the compressed and purified feed air stream to the auxiliary heat exchanger;   diverting part of the first refrigeration stream from the first refrigeration circuit to the auxiliary heat exchanger and warming the diverted portion of the first refrigeration stream in the auxiliary heat exchanger via indirect heat exchange with diverted fourth portion of the compressed and purified feed air stream;   directing the fourth portion of the compressed and purified feed air stream exiting the auxiliary heat exchanger to the distillation column system;   directing a remaining portion of the first refrigeration stream to a lower pressure column of the distillation column system to impart a first portion of the refrigeration required by the distillation column system; and   directing the cooled second refrigeration stream to the higher pressure column of the distillation column system to impart a second portion of the refrigeration required by the distillation column system.   
     
     
         2 . The method of  claim 1  further comprising the steps of:
 further compressing the first portion of the compressed and purified feed air stream within the first refrigeration circuit; 
 cooling the further compressed first portion of the compressed and purified feed air stream; and 
 expanding the further compressed first portion of the compressed and purified feed air stream in a first turbo-expander disposed within the first refrigeration circuit to produce the first refrigeration stream. 
 
     
     
         3 . The method of  claim 2  wherein the step of cooling the further compressed first portion of the compressed and purified feed air stream further comprises cooling the further compressed first portion of the compressed and purified feed air stream in an aftercooler. 
     
     
         4 . The method of  claim 2  wherein the step of cooling the further compressed first portion of the compressed and purified feed air stream further comprises partially cooling the further compressed first portion of the compressed and purified feed air stream in the main heat exchanger. 
     
     
         5 . The method of  claim 1  wherein the step of directing the cooled fourth portion of the compressed and purified feed air stream to the distillation column system further comprises directing the cooled fourth portion of the compressed and purified feed air stream to the higher pressure column of the distillation column system. 
     
     
         6 . The method of  claim 1  further comprising the steps of:
 diverting a portion of the second refrigeration stream from the auxiliary refrigeration circuit to the first refrigeration circuit; and 
 combining the diverted portion of the second refrigeration stream with the first portion of the compressed and purified feed air stream in the first refrigeration circuit. 
 
     
     
         7 . The method of  claim 6  further comprising the steps of:
 diverting a portion of the second refrigeration stream that is partially cooled from the auxiliary heat exchanger in the auxiliary refrigeration circuit to the first refrigeration circuit; and 
 combining the diverted portion of the second refrigeration stream with the first portion of the compressed and purified feed air stream in the first refrigeration circuit upstream of the turbo-expander. 
 
     
     
         8 . The method of  claim 1  wherein the step of diverting a fraction of the further compressed feed air stream from the booster air compression circuit to the auxiliary refrigeration circuit further comprises:
 further compressing the third portion of the compressed and purified feed air stream in a plurality of compression stages; and 
 diverting a first fraction of the third portion of the compressed and purified feed air stream from an interstage location of the plurality of compression stages to the auxiliary refrigeration circuit. 
 
     
     
         9 . The method of  claim 1  wherein the step of diverting a fraction of the further compressed feed air stream from the booster air compression circuit to the auxiliary refrigeration circuit further comprises:
 further compressing the third portion of the compressed and purified feed air stream in a plurality of compression stages; and 
 diverting one or more fractions of the third portion of the compressed and purified feed air stream from one or more interstage locations of the plurality of compression stages to the auxiliary refrigeration circuit; 
 controlling the flow of the diverted one or more fractions of the third portion of the compressed and purified feed air stream with one or more flow control valves disposed between the booster air compression circuit and the second turbo-expander in the auxiliary refrigeration circuit; 
 wherein the inlet pressure to the second turbo-expander in the auxiliary refrigeration circuit is controlled by adjusting the one or more flow control valves which in turn controls the second portion of the refrigeration required by the distillation column system. 
 
     
     
         10 . The method of  claim 1  wherein the auxiliary refrigeration circuit further comprises an auxiliary compressor, the second turbo-expander and the auxiliary heat exchanger and wherein the method further comprises the steps of:
 diverting the fraction of the further compressed feed air stream from the booster air compression circuit to the auxiliary compressor; 
 further compressing the diverted fraction of the compressed feed air stream from the booster air compression circuit; 
 partially cooling the further compressed diverted fraction in the auxiliary heat exchanger via indirect heat exchange with the diverted portion of the first refrigeration stream; 
 expanding the partially cooled further compressed diverted fraction in the second turbo-expander; 
 further cooling the expanded diverted fraction in the auxiliary heat exchanger via indirect heat exchange with the diverted portion of the first refrigeration stream to produce the cooled second refrigeration stream; and 
 directing the cooled second refrigeration stream to the higher pressure column of the distillation column system to impart the second portion of the refrigeration required by the distillation column system. 
 
     
     
         11 . An air separation unit configured to produce at least one liquid product stream, the air separation unit comprising:
 an incoming air compression and purification train configured to produce a compressed and purified feed air stream;   a primary refrigeration circuit having a first turbo-expander, the primary refrigeration circuit operatively coupled to the incoming air compression and purification train and configured to receive a first portion of the compressed and purified feed air stream and expand the first portion of the compressed and purified feed air stream in the first turbo-expander to produce a first cooled refrigeration stream;   a main heat exchanger operatively coupled to the incoming air compression and purification train and configured to receive a second portion of the compressed and purified feed air stream and to cool the second portion of the compressed and purified feed stream to a temperature suitable for the rectification of the compressed and purified feed air stream;   a booster air compression circuit operatively coupled to the incoming air compression and purification train and the main heat exchanger, the booster air compression circuit configured to receive a third portion of the compressed and purified feed air stream, further compress the third portion and direct the further compressed third portion to the main heat exchanger to produce a liquid air stream;   a second turbo-expander configured to receive a fraction of the further compressed third portion and expand the fraction of the further compressed third portion to produce a second refrigeration stream; and   an auxiliary heat exchanger operatively coupled to the incoming air compression and purification train, the booster air compression circuit and the primary refrigeration circuit, the auxiliary heat exchanger configured to receive a fourth portion of the compressed and purified feed air stream and cool the fourth portion of the compressed and purified feed air stream via indirect heat exchange with the second refrigeration stream and a diverted portion of the first refrigeration stream;   a distillation column system operatively coupled to the primary refrigeration circuit, the booster air compression circuit and the auxiliary heat exchanger, the distillation column system configured to rectifying some or all of the first refrigeration stream, the second refrigeration stream, the liquid air stream, and the cooled second portion of the compressed and purified feed air stream by a cryogenic rectification process to produce the at least one liquid product stream.   
     
     
         12 . The air separation unit of  claim 11  wherein the primary refrigeration circuit further comprises a compressor configured for further compressing the first portion of the compressed and purified feed air stream within the primary refrigeration circuit; and wherein the compressor is operatively coupled to the main heat exchanger such that the further compressed the first portion of the compressed and purified feed air stream is partially cooled in the main heat exchanger. 
     
     
         13 . The air separation unit of  claim 11  wherein the cooled fourth portion of the compressed and purified feed air stream exiting the auxiliary heat exchanger is directed to a higher pressure column of the distillation column system. 
     
     
         14 . The air separation unit of  claim 11  further comprising a recycle circuit connecting the auxiliary heat exchanger with the primary refrigeration circuit wherein a portion of the second refrigeration stream is recycled to the first refrigeration circuit. 
     
     
         15 . The air separation unit of  claim 14  wherein the portion of the second refrigeration stream is recycled to the first refrigeration circuit is partially cooled within the auxiliary heat exchanger and is recycled to a location in the first refrigeration circuit upstream of the first turbo-expander. 
     
     
         16 . The air separation unit of  claim 11  further comprising an auxiliary refrigeration circuit that includes an auxiliary compressor configured to receive the fraction of the further compressed feed air stream diverted from the booster air compression circuit, the second turbo-expander configured to receive a compressed air stream from the auxiliary compressor and expand the compressed air stream, and the auxiliary heat exchanger configured to receive the expanded air stream from the second turbo-expander. 
     
     
         17 . The air separation unit of  claim 11  wherein the booster air compression circuit further comprises a plurality of compression stages and a diversion circuit for diverting one or more fractions of the further compressed feed air stream from one or more interstage locations of the plurality of compression stages to the auxiliary refrigeration circuit. 
     
     
         18 . The air separation unit of  claim 11  further comprising one or more flow control valves disposed between the booster air compression circuit and the second turbo-expander in the auxiliary refrigeration circuit and configured for controlling the flow of the diverted one or more fractions of the third portion of the compressed and purified feed air stream.