Dynamic control of reflux rate in hydrocarbon distillation

Abstract

A reflux rate of a demethanizer is dynamically controlled. An ethane content of a hydrocarbon feed stream, a residue stream, and a bottoms stream are used by a controller to determine an ethane recovery value. The hydrocarbon feed stream includes methane, ethane, and propane and is fractionated by the demethanizer. The residue stream includes ethane. The bottoms stream includes propane. The controller controls the flow of reflux to the demethanizer, such that the determined ethane recovery value is equal to or greater than a specified target ethane recovery set point. The controller determines an objective value as a ratio of power consumption to the determined ethane recovery value. The controller controls the amount of cooling provided to the hydrocarbon feed stream, thereby adjusting power consumption and reducing the objective value while maintaining the ethane recovery value at or above the specified target ethane recovery set point.

What is claimed is: 
     
         1 . A method for dynamically controlling a reflux rate of a demethanizer, the method comprising:
 receiving, by a controller, an ethane content of each of a hydrocarbon feed stream, a residue stream, and a bottoms stream, wherein the hydrocarbon feed stream comprises methane, ethane, and propane, wherein the residue stream comprises at least a portion of the ethane from the hydrocarbon feed stream, wherein the bottoms stream comprises at least a portion of the propane from the hydrocarbon feed stream, wherein the hydrocarbon feed stream enters and is fractionated by the demethanizer;   determining, by the controller, an ethane recovery value at least based on the received ethane contents of the hydrocarbon feed stream, the residue stream, and the bottoms stream;   receiving, by the controller, a flow rate of reflux provided to the demethanizer;   transmitting, by the controller, a flow signal to a reflux flow control valve to adjust the flow rate of reflux provided to the demethanizer, such that the determined ethane recovery value is equal to or greater than a specified target ethane recovery set point;   receiving, by the controller, a power consumption of a refrigeration cycle providing cooling to the hydrocarbon feed stream;   determining, by the controller, an objective value as a ratio of the determined power consumption to the determined ethane recovery value; and   after adjusting the flow rate of reflux provided to the demethanizer, transmitting, by the controller, a signal to reduce power consumed by the refrigeration cycle to reduce the objective value, while maintaining the determined ethane recovery value at or above the specified target ethane recovery set point.   
     
     
         2 . The method of  claim 1 , wherein determining the ethane recovery value comprises calculating the ethane recovery value as: 
       
         
           
             
               
                 Ethane 
                 ⁢ 
                     
                 Recovery 
               
               = 
               
                 1 
                 - 
                 
                   
                     
                       
                         Ethane 
                         ⁢ 
                             
                         content 
                         ⁢ 
                             
                         of 
                         ⁢ 
                             
                         Residue 
                         ⁢ 
                             
                         Gas 
                       
                       + 
                       
                         Ethane 
                         ⁢ 
                             
                         content 
                         ⁢ 
                             
                         of 
                         ⁢ 
                             
                         Bottoms 
                       
                     
                     
                       Ethane 
                       ⁢ 
                           
                       content 
                       ⁢ 
                           
                       Hydrocarbon 
                       ⁢ 
                           
                       Feed 
                     
                   
                   . 
                 
               
             
           
         
       
     
     
         3 . The method of  claim 2 , wherein the refrigeration cycle comprises a compressor and a refrigerant flowing through and being pressurized by the compressor, and the transmitted signal causes a flow rate of the refrigerant flowing through and being pressurized by the compressor to reduce, thereby reducing the power consumed by the refrigeration cycle. 
     
     
         4 . The method of  claim 3 , comprising fractionating, by the demethanizer, the hydrocarbon feed stream, wherein fractionating the hydrocarbon feed stream comprises:
 separating the hydrocarbon feed stream into a vapor phase and a liquid phase;   flowing the liquid phase to the demethanizer as feed;   flowing a first portion of the vapor phase through a turboexpander;   flowing the first portion of the vapor phase from the turboexpander to the demethanizer as feed; and   flowing a second portion of the vapor phase to the demethanizer as reflux.   
     
     
         5 . The method of  claim 4 , wherein fractionating the hydrocarbon feed stream comprises transferring heat from the residue gas stream exiting the demethanizer to the second portion of the vapor phase entering the demethanizer. 
     
     
         6 . The method of  claim 4 , wherein fractionating the hydrocarbon feed stream comprises generating, by the turboexpander, electrical power in response to the first portion of the vapor phase flowing through and expanding across the turboexpander, and at least a portion of the electrical power generated by the turboexpander is provided to the compressor to pressurize the refrigerant. 
     
     
         7 . A method comprising:
 transferring, by a cooler, heat from a hydrocarbon feed stream to a refrigerant cycling through a refrigeration cycle, wherein the hydrocarbon feed stream comprises methane, ethane, and propane;   fractionating, by a distillation unit comprising a demethanizer and a de-ethanizer, the hydrocarbon feed stream to produce a residue gas stream, an ethane product stream, and a bottoms stream, wherein the residue gas stream is produced by the demethanizer and comprises at least a portion of the methane, the ethane product stream is produced by the de-ethanizer and comprises at least a portion of the ethane, the bottoms stream is produced by the de-ethanizer and comprises at least a portion of the propane, and fractionating the feed stream comprises:
 separating the hydrocarbon feed stream into a vapor phase and a liquid phase; 
 flowing the liquid phase to the demethanizer as feed; 
 flowing a first portion of the vapor phase through a turboexpander; 
 generating, by the turboexpander, electrical power in response to the first portion of the vapor phase flowing through and expanding across the turboexpander; 
 flowing the first portion of the vapor phase from the turboexpander to the demethanizer as feed; 
 at least partially condensing a second portion of the vapor phase; 
 flowing the at least partially condensed second portion to the demethanizer as reflux; and 
 flowing a liquid stream from the demethanizer to the de-ethanizer as feed; 
   determining an objective value as a ratio of power consumption by the refrigeration cycle to ethane recovery by the distillation unit; and   adjusting a flow rate of the at least partially condensed second portion flowing to the demethanizer as reflux, a flow rate of the refrigerant flowing through the cooler, or both to minimize the objective value while maintaining the ethane recovery at or above a specified target ethane recovery set point.   
     
     
         8 . The method of  claim 7 , comprising measuring an ethane content of each of the hydrocarbon feed stream, the residue stream produced by the demethanizer, and the bottoms stream produced by the de-ethanizer. 
     
     
         9 . The method of  claim 8 , wherein the ethane recovery value is calculated as: 
       
         
           
             
               
                 Ethane 
                 ⁢ 
                     
                 Recovery 
               
               = 
               
                 1 
                 - 
                 
                   
                     
                       
                         Ethane 
                         ⁢ 
                             
                         content 
                         ⁢ 
                             
                         of 
                         ⁢ 
                             
                         Residue 
                         ⁢ 
                             
                         Gas 
                       
                       + 
                       
                         Ethane 
                         ⁢ 
                             
                         content 
                         ⁢ 
                             
                         of 
                         ⁢ 
                             
                         Bottoms 
                       
                     
                     
                       Ethane 
                       ⁢ 
                           
                       content 
                       ⁢ 
                           
                       Hydrocarbon 
                       ⁢ 
                           
                       Feed 
                     
                   
                   . 
                 
               
             
           
         
       
     
     
         10 . The method of  claim 9 , wherein adjusting the flow rate of the refrigerant flowing through the cooler comprises reducing the flow rate of the refrigerant flowing through the cooler, thereby reducing the power consumption by the refrigeration cycle. 
     
     
         11 . The method of  claim 10 , wherein fractionating the feed hydrocarbon stream comprises transferring heat from the residue gas stream exiting the demethanizer to the second portion of the vapor phase entering the demethanizer. 
     
     
         12 . The method of  claim 10 , wherein at least a portion of the electrical power generated by the turboexpander is used to pressurize the refrigerant cycling through the refrigeration cycle. 
     
     
         13 . A system comprising:
 a demethanizer configured to fractionate a hydrocarbon feed stream based on volatility to produce a residue gas stream and a bottoms stream, wherein the hydrocarbon feed stream comprises methane, ethane, and propane, the residue gas stream comprises at least a portion of the methane, and the bottoms stream comprises at least a portion of the ethane and at least a portion of the propane;   a refrigeration cycle comprising a refrigerant and a cooler, wherein the cooler is configured to transfer heat from the hydrocarbon feed stream to the refrigerant to provide cooling to the hydrocarbon feed stream upstream of the demethanizer;   a reflux flow control valve configured to control a flow rate of reflux provided to the demethanizer; and   a controller communicatively coupled to the reflux flow control valve and to the refrigeration cycle, wherein the controller is configured to:
 determine an objective value as a ratio of power consumption by the refrigeration cycle to ethane recovery by the de-ethanizer; and 
 transmit a reflux signal to the reflux flow control valve to adjust the flow rate of the reflux provided to the demethanizer, a refrigeration signal to the refrigeration cycle to adjust a flow rate of the refrigerant flowing through the cooler, or both to minimize the objective value while maintaining the ethane recovery at or above a specified target ethane recovery set point. 
   
     
     
         14 . The system of  claim 13 , comprising:
 a knockout drum downstream of the cooler and upstream of the demethanizer, wherein the knockout drum is configured to receive the hydrocarbon feed stream from the cooler and separate the hydrocarbon feed stream into a vapor phase and a liquid phase;   a turboexpander configured to receive a first portion of the vapor phase, wherein the turboexpander is configured to generate electrical power in response to the first portion of the vapor phase expanding through the turboexpander, wherein the demethanizer is configured to receive the first portion of the vapor phase from the turboexpander as feed; and   a de-ethanizer configured to receive the bottoms stream as feed, wherein the de-ethanizer is configured to fractionate the bottoms stream based on volatility to produce an ethane product stream and a propane product stream, wherein the ethane product stream comprises at least a portion of the ethane from the hydrocarbon feed stream, wherein the propane product stream comprises at least a portion of the propane from the hydrocarbon feed stream.   
     
     
         15 . The system of  claim 14 , comprising a cross exchanger configured to transfer heat between the residue gas exiting the demethanizer and the second portion of the vapor phase entering the demethanizer. 
     
     
         16 . The system of  claim 15 , comprising:
 a first composition analyzer configured to determine an ethane content of the hydrocarbon feed stream;   a second composition analyzer configured to determine an ethane content of the residue stream; and   a third composition analyzer configured to determine an ethane content of the propane product stream.   
     
     
         17 . The system of  claim 16 , wherein the controller is configured to determine the ethane recovery as: 
       
         
           
             
               
                 Ethane 
                 ⁢ 
                     
                 Recovery 
               
               = 
               
                 1 
                 - 
                 
                   
                     
                       
                         Ethane 
                         ⁢ 
                             
                         content 
                         ⁢ 
                             
                         of 
                         ⁢ 
                             
                         Residue 
                         ⁢ 
                             
                         Gas 
                       
                       + 
                       
                         Ethane 
                         ⁢ 
                             
                         content 
                         ⁢ 
                             
                         of 
                         ⁢ 
                             
                         Bottoms 
                       
                     
                     
                       Ethane 
                       ⁢ 
                           
                       content 
                       ⁢ 
                           
                       Hydrocarbon 
                       ⁢ 
                           
                       Feed 
                     
                   
                   . 
                 
               
             
           
         
       
     
     
         18 . The system of  claim 17 , wherein the refrigeration cycle comprises a refrigerant flow control valve, and the controller is configured to transmit the refrigeration signal to the refrigerant flow control valve to reduce the flow rate of the refrigerant flowing through the cooler, thereby reducing power consumed by the refrigeration cycle. 
     
     
         19 . The system of  claim 17 , wherein the refrigeration cycle comprises a compressor configured to pressurize the refrigerant, and the turboexpander is coupled to the compressor for providing at least a portion of the generated electrical power to the compressor for pressurizing the refrigerant. 
     
     
         20 . The system of  claim 17 , comprising a pressure sensor configured to measure a pressure of the hydrocarbon feed stream and a temperature sensor configured to measure a temperature of the hydrocarbon feed stream, wherein the power consumption by the refrigeration cycle depends at least on the ethane content of the hydrocarbon feed stream, the pressure of the hydrocarbon feed stream, and the temperature of the hydrocarbon feed stream.