Optimization of a dual refrigeration system natural gas liquid plant via empirical experimental method

Abstract

The current invention is an empirical optimization method based on statistical modeling relating NGL plant process variables with the refrigeration system's electricity usage. The method identifies the key process control variables in an NGL plant to be optimized. This method is applicable to an NGL plant that uses dual refrigeration systems.

1 . A method for optimizing the production of NGL outlet stream from an NGL plant, the method comprising the steps of: 
 feeding a natural gas feed stream to a first chilling unit to produce a chilled rich gas stream and a chilled liquid stream;    feeding the chilled rich gas stream to a second chilling unit to produce a second chilled rich gas stream and a second chilled liquid stream;    feeding the second chilled rich gas stream to a third chilling unit to produce a third chilled liquid stream;    feeding the chilled liquid stream and the second chilled liquid stream and the third chilled liquid stream to a demethanizer column, the demethanizer column producing an overhead stream and a bottoms stream, the bottoms stream having a bottom product specification, the overhead stream defining an overhead propane concentration;    feeding the overhead stream through an overhead valve having an overhead valve outlet pressure;    providing heat exchange through a first propane refrigeration system to the first chilling unit, the first chilling unit having a first chiller, the first chilling unit having a first chill down separator, the first propane refrigeration system having a propane compressor, the propane compressor defining a propane compressor power output and a propane compressor suction pressure,    providing heat exchange through a second propane refrigeration system operable for providing cooling to the second chilling unit, the second chilling unit having a second chill down separator, the second chilling unit including a primary second chiller, the second propane refrigeration system including a second propane compressor defining a second propane compressor power output and a second propane compressor suction pressure;    providing heat exchange to the third chilling unit through an ethane refrigeration system having an ethane compressor, the ethane compressor defining an ethane compressor suction pressure; and    minimizing a refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range and while maintaining the overhead propane concentration within a predetermined overhead propane concentration range, the refrigeration load being the electricity required to operate the first propane refrigeration system, the second propane refrigeration system and the ethane refrigeration system.    
   
   
       2 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the propane compressor power output within a predetermined propane compressor power output range.  
   
   
       3 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the propane compressor suction pressure within a predetermined propane compressor suction pressure range.  
   
   
       4 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining a propane compressor scraper output pressure within a predetermined propane compressor output pressure range.  
   
   
       5 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the first tray temperature within a predetermined first tray temperature range.  
   
   
       6 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the overhead valve output pressure of the overhead stream within a predetermined overhead valve output pressure range.  
   
   
       7 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the first chiller output level within a predetermined first chiller output level range.  
   
   
       8 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the ethane compressor suction pressure within a predetermined ethane compressor suction pressure range.  
   
   
       9 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the ethane compressor power output within a predetermined ethane compressor power output range.  
   
   
       10 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining an overhead temperature of the overhead stream within a predetermined overhead temperature range.  
   
   
       11 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the first chill down separator temperature within a predetermined first chill down separator temperature range.  
   
   
       12 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining a second propane exchanger temperature of a second propane exchanger within a predetermined second propane exchanger temperature range.  
   
   
       13 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining a second propane compressor suction pressure within a predetermined second propane compressor suction pressure range.  
   
   
       14 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the second propane compressor power output within a predetermined second propane power output range.  
   
   
       15 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the primary second chiller output level within a predetermined primary second chiller output level.  
   
   
       16 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the subsequent second chiller output level within a predetermined subsequent second chiller output level range.  
   
   
       17 . The method of  claim 1  wherein the step of minimizing the refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range further comprises the step of maintaining the second chill down separator within a second chill down separator temperature range.  
   
   
       18 . A liquefied natural gas plant for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant comprising: 
 a first chilling unit for cooling at least a portion of the inlet gas feed stream by heat exchange contact with first and second expanded refrigerants to produce from the first chilling unit a chilled rich gas stream and a chilled liquid stream, the first chilling unit having a first chiller, the first chiller defining a first chiller output level, and the first chiller unit having a first chill down separator, the first chill down separator defining a first chill down separator temperature;    a second chilling unit to receive chilled rich gas stream and to further chill the chilled rich gas stream to produce a second chilled rich gas stream and a second chilled liquid stream, the second chilling unit comprising a second chill down separator defining a second chill down separator temperature, a subsequent second chiller defining a subsequent second chiller output level, and a primary second chiller;    a third chilling unit to receive second chilled rich gas stream and further chill second chilled rich gas to produce a third chilled liquid stream;    a demethanizer column for receiving the chilled liquid stream and the second chilled liquid stream and the third chilled liquid stream, the demethanizer column producing an overhead stream and a bottoms stream, the demethanizer column having a top tray in an upper section of the demethanizer column and a mid-tray in a middle section of the demethanizer column, the top tray having a top tray temperature, the bottoms stream having a bottom ratio defined by methane concentration of the bottom stream divided by ethane concentration of the bottom stream, the overhead stream defining an overhead propane concentration;    an overhead valve receiving the overhead stream, the overhead valve having an overhead valve outlet pressure;    a first propane refrigeration system operable to provide heat exchange with the first chilling unit, the first propane refrigeration system having a propane compressor, the propane compressor defining a propane compressor power output and a propane compressor suction pressure,    a second propane refrigeration system operable to provide heat exchange to the second chilling unit, the second propane refrigeration system including a second propane compressor;    an ethane refrigeration system operable to provide heat exchange to the third chilling unit, the ethane refrigeration system having an ethane compressor, the ethane compressor defining an ethane compressor suction pressure; and    means for minimizing a refrigeration load while maintaining the bottom ratio within a predetermined bottom ratio range and while maintaining the overhead propane concentration within a predetermine overhead propane concentration range, the refrigeration load being the electricity required to operate the first propane refrigeration system, the second propane refrigeration system and the ethane refrigeration system.    
   
   
       19 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the propane compressor power output within a predetermined propane compressor power output range.  
   
   
       20 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the propane compressor suction pressure within a predetermined propane compressor suction pressure range.  
   
   
       21 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining a propane compressor scraper output pressure within a predetermined propane compressor output pressure range.  
   
   
       22 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the first tray temperature within a predetermined first tray temperature range.  
   
   
       23 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the overhead valve output pressure of the overhead stream within a predetermined overhead valve output pressure range.  
   
   
       24 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the first chiller output level within a predetermined first chiller output level range.  
   
   
       25 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the ethane compressor suction pressure within a predetermined ethane compressor suction pressure range.  
   
   
       26 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining ethane compressor power output within a predetermined ethane compressor power output range.  
   
   
       27 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining an overhead temperature of the overhead stream within a predetermined overhead temperature range.  
   
   
       28 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the first chill down separator temperature within a predetermined first chill down separator temperature range.  
   
   
       29 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining a second propane exchanger temperature of a second propane exchanger within a predetermined second propane exchanger temperature range.  
   
   
       30 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining a second propane compressor suction pressure within a predetermined second propane compressor suction pressure range.  
   
   
       31 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the second propane compressor power output within a predetermined second propane compressor power output range.  
   
   
       32 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the primary second chiller output level within a predetermined primary second chiller output level.  
   
   
       33 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the subsequent second chiller output level within a predetermined subsequent second chiller output level range.  
   
   
       34 . The liquefied natural gas plant of  claim 18  for maximizing the production of NGL from an inlet gas feed stream, the liquefied natural gas plant further comprising means for maintaining the second chill down separator  90  within a second chill down separator temperature range.