Method and apparatus for improving start-up for an air separation apparatus

Abstract

A method for operating an air separation unit having both a startup phase and a steady-state operating phase is provided. The method can include the steps of: introducing a plurality of air streams into a cold box, wherein each stream is cooled in a heat exchanger and then sent to a system of distillation columns for separation therein by cryogenic distillation; producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger; determining whether each stream of the plurality of air streams is a net cold producing stream or net cold consuming streams; and adjusting a flow rate of each stream of the plurality of air streams during the startup phase such that each stream that is a net cold producing stream has a higher flow rate during the startup phase as compared to the steady-state operating phase, and wherein each stream that is a net cold consuming stream has a lower flow rate during the startup phase as compared to the steady-state operating phase.

What is claimed is: 
     
         1 . A method for operating an air separation unit having both a startup phase and a steady-state operating phase, the air separation unit having a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns, wherein the method, for both the startup phase and the steady-state operating phase, comprises the steps of:
 splitting a purified and compressed air stream downstream of the main air compressor into a first air stream and a second air stream;   cooling the first air stream in a heat exchanger and then introducing the first air stream into the system of distillation columns for separation therein by cryogenic distillation;   boosting the pressure of the second air stream in the booster air compressor to form a boosted air stream;   expanding at least a first portion of the boosted air stream to provide cooling energy, and then introducing the resulting expanded air stream to the system of distillation columns for separation therein by cryogenic distillation; and   producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger;   wherein during the start-up phase, the first air stream has a start-up flow rate (Q FAS1 ) and the boosted air stream has a start-up flow rate (Q BAS1 ),   wherein during the steady-state operating phase, the first air stream has a steady state flow rate (Q FAS2 ) and the boosted air stream has a steady state flow rate (Q BAS2 ),   wherein the flow rate of the boosted air stream is set higher during the start-up phase as compared to during the steady-state operating phase, such that Q BAS1 >Q BAS2 , and   wherein the flow rate of the first air stream is set lower during the start-up phase as compared to during the steady-state operating phase, such that Q FAS1 <Q FAS2 .   
     
     
         2 . The method of  claim 1 , further comprising the steps of splitting the boosted air stream into the first portion and a second portion, wherein the first portion is expanded in a cold turbine after partial cooling in the heat exchanger before being sent to the system of distillation columns for separation therein by cryogenic distillation. 
     
     
         3 . The method of  claim 2 , further comprising the steps of further compressing the second portion of the boosted air stream in a warm compressor, cooling the second portion of the boosted air stream in the heat exchanger and then expanding the second portion of the boosted air stream in a Joule-Thompson valve before introduction into the system of distillation column for separation therein by cryogenic distillation. 
     
     
         4 . A method for operating an air separation unit having both a startup phase and a steady-state operating phase, the air separation unit having a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns, wherein the method comprises the steps of:
 introducing a plurality of air streams into the cold box, wherein each stream is cooled in the heat exchanger and then sent to the system of distillation columns for separation therein by cryogenic distillation;   producing a plurality of air gas streams from the system of distillation columns and warming said plurality of air gas streams in the heat exchanger; and   determining whether each stream of the plurality of air streams is a net cold producing stream or a net cold consuming stream,   wherein a flow ratio between the net cold producing streams and the net cold consuming streams is increased during the startup phase as compared to the steady-state operating phase.   
     
     
         5 . The method of  claim 4 , wherein the flow ratio between the net cold producing streams and the net cold consuming streams is increased during the startup phase as compared to the steady-state operating phase by increasing a flow rate of each stream of the plurality of air streams that is a net cold producing stream during the startup phase as compared to the steady-state operating phase and decreasing a flow rate of each stream of the plurality of air streams that is a net cold consuming stream during the startup phase as compared to the steady-state operating phase. 
     
     
         6 . An apparatus for operating an air separation unit having both a startup phase and a steady-state operating phase, the air separation unit having a main air compressor, a booster air compressor, and a cold box having a heat exchanger, and a system of distillation columns,
 wherein the cold box is configured to receive a plurality of air streams and cool each stream in the heat exchanger,   wherein the system of distillation columns is in fluid communication with the heat exchanger, wherein the system of distillation columns is configured to produce a plurality of air gas streams,   wherein the apparatus further comprises:   means for determining whether each stream of the plurality of air streams is a net cold producing stream or a net cold consuming stream; and   means for increasing a flow ratio between the net cold producing streams and the net cold consuming streams during the startup phase as compared to the steady-state operating phase.