Method for controlling the compression of an incoming feed air stream to a cryogenic air separation plant

Abstract

A method for controlling the compression of an incoming feed air stream to a cryogenic air separation plant using at least two variable speed compressor drive assemblies controlled in tandem is provided. The first variable speed driver assembly drives at least one compression stage in the lower pressure compressor unit driven while the second variable speed driver assembly drives higher pressure compression stage disposed either in the common air compression train or the split functional compression train of the air separation plant. The first and second variable speed driver assemblies are preferably high speed, variable speed electric motor assemblies each having a motor body, a motor housing, and a motor shaft with one or more impellers directly and rigidly coupled to the motor shaft via a sacrificial rigid shaft coupling.

What is claimed is: 
     
         1 . A method for controlling the compression of an incoming feed stream to a cryogenic air separation plant, the method comprising the steps of:
 (a) compressing at least a portion of the incoming feed stream in a lower pressure single stage or multi-stage compression unit of the common air compression train, at least one compression stage in the lower pressure single stage or multi-stage compression unit driven directly by a first variable speed driver assembly;   (b) further compressing the compressed stream in one or more higher pressure single stage or multi-stage compression units of the common air compression train, wherein the at least one of the higher pressure single stage or multi-stage compression units are driven by a second variable speed driver assembly; and   (c) purifying the further compressed feed stream to remove impurities either after step (a); after step (b) or between compression stages of step (b);   wherein a volumetric flow of the feed stream is controlled by adjusting the speed of a primary driver assembly selected from the first variable speed driver assembly or the second variable speed driver assembly in response to changes in the operating conditions of the cryogenic air separation plant and wherein a ratio of the speed of the variable speed driver assemblies prior to such adjustment is different than the ratio of the speed of the variable speed driver assemblies after adjustment; and   whereby the efficiency of the cryogenic air separation plant changes by adjusting the speed of the first variable speed driver assembly or the second variable speed driver assembly or both in response to changes in the operating conditions of the cryogenic air separation plant.   
     
     
         2 . The method of  claim 1  wherein the efficiency of the cryogenic air separation plant increases by adjusting the speed of the variable speed driver assemblies. 
     
     
         3 . The method of  claim 1  further comprising the step of directing portions of the compressed and purified feed air stream to a split functional air compression train having one or more booster compression stages. 
     
     
         4 . The method of  claim 3  wherein the speed of the first variable speed driver assembly is set in response to a measured flow rate of air in the common air compression train and wherein the speed of the second variable speed driver assembly is set in response to a measured pressure of at least one of the portions of purified, compressed air streams in the split functional air compression train and the speed of the first variable speed driver assembly. 
     
     
         5 . The method of  claim 1  wherein the speed of the first variable speed driver assembly is set in response to a measured flow rate of air in the common air compression train and wherein the speed of the second variable speed driver assembly is set in response to a discharge pressure in the common air compression train and the speed of the first variable speed driver assembly. 
     
     
         6 . The method of  claim 1  wherein the speed of the first variable speed driver assembly is set in response to the measured flow rate of air in the common air compression train and one or more process limits and wherein the speed of the second variable speed driver assembly is set in response to the one or more one or more process limits and the speed of the first variable speed driver assembly. 
     
     
         7 . The method of  claim 1  wherein the speed of the first variable speed driver assembly is set in response to the measured flow rate of air in the common air compression train and one or more compression stage limits and wherein the speed of the second variable speed driver assembly is set in response to the one or more compression stage limits and the speed of the first variable speed driver assembly. 
     
     
         8 . The method of  claim 1  wherein the speed of the first variable speed driver assembly is set in response to the measured flow rate of air in the common air compression train and one or more driver assembly limits and wherein the speed of the second variable speed driver assembly is set in response to the one or more one or more driver assembly limits and the speed of the first variable speed driver assembly. 
     
     
         9 . The method of  claim 1  wherein the speed of the first variable speed driver assembly is set in response to the measured flow rate of air in the common air compression train and one or more limits selected from the group of process limits, compression stage limits, and driver assembly limits, and wherein the speed of the second variable speed driver assembly is set in response to the speed of the first variable speed driver assembly and one or more limits selected from the group of process limits, compression stage limits, and driver assembly limits. 
     
     
         10 . The method of  claim 1  wherein the speed of the first variable speed driver assembly or the speed of the second variable speed driver assembly or both are further adjusted periodically in response to the diversion or venting of a portion of the compressed air from the common air compression train. 
     
     
         11 . The method of  claim 1  wherein the speed of the first variable speed driver assembly or the speed of the second variable speed driver assembly or both are further adjusted in response to changes in ambient air conditions. 
     
     
         12 . The method of  claim 1  wherein the speed of the second variable speed driver assembly is adjusted in response to changes in the operating conditions of the cryogenic air separation plant and the speed of the first variable speed driver assembly and wherein a ratio of the speed of the variable speed driver assemblies prior to such adjustment is different than the ratio of the speed of the variable speed driver assemblies after adjustment. 
     
     
         13 . A method for controlling the compression of a feed air stream to a cryogenic air separation plant having a common air compression train and a split functional air compression train, the method comprising the steps of:
 (a) compressing at least a portion of an incoming feed stream in a lower pressure single stage or multi-stage compressor of the common air compression train, at least one compression stage in the lower pressure single stage or multi-stage compressor driven directly by a first variable speed driver assembly; and   (b) further compressing the compressed stream in one or more higher pressure single stage or multi-stage compressors of the common air compression train wherein the at least one of the higher pressure single stage or multi-stage compression units are driven by a second variable speed driver assembly;   wherein a volumetric flow of the feed stream is controlled by adjusting the speed of a primary driver assembly selected from the first variable speed driver assembly or the second variable speed driver assembly in response to changes in the operating conditions of the cryogenic air separation plant;   wherein the efficiency of the air separation plant at the volumetric flow is optimized by managing one or more operating pressures within the plant including one or more discharge pressures in the common air compression train or in the split functional air compression train, and the one or more operating pressures change by adjusting the speed of the first variable speed driver assembly or the second variable speed driver assembly or both in response to changes in the operating conditions of the cryogenic air separation plant and the speed of the primary driver assembly; and   wherein during turndown conditions a ratio of the speed of the first variable speed driver assembly to the speed of the second variable speed driver assembly is between about 0.75 to 0.99.   
     
     
         14 . The method of  claim 13 , further comprising the steps of:
 further compressing the compressed stream after step (a) in one or more intermediate pressure compressors of the common air compression train wherein the one or more of the intermediate pressure compressors are driven by a third variable speed driver assembly; and   wherein the speed of the third variable speed driver assembly is adjusted, in part, in response to the speed of the first and second variable speed driver assemblies.   
     
     
         15 . The method of  claim 13 , further comprising the steps of:
 further compressing the compressed stream after step (b) in one or more higher pressure compressors of the split functional air compression train wherein the one or more of the higher pressure compressors of the split functional air compression train are driven by a third variable speed driver assembly; and   wherein the speed of the third variable speed driver assembly is adjusted, in part, in response to the speed of the first and second variable speed driver assemblies.   
     
     
         16 . The method of  claim 13 , further comprising the steps of:
 directing all or a portion of the feed air stream into inlet guide vanes before step (a), the inlet guide vanes being operatively associated with the lower pressure compressor; and   wherein the position of the inlet guide vanes are adjusted, in part, in response to the speed of the first and second variable speed driver assemblies.